Telesis Bio 10-Q 5 Nov 20 2020 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Nov. 02, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	001-38550
Entity Registrant Name	Translate Bio, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	61-1807780
Entity Address, Address Line One	29 Hartwell Avenue
Entity Address, City or Town	Lexington
Entity Address, Postal Zip Code	02421
City Area Code	617
Local Phone Number	945-7361
Trading Symbol	TBIO
Entity Central Index Key	0001693415
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Address, State or Province	MA
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		74,321,827

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 674,050	$ 84,580
Short-term investments		104,098
Collaboration receivables	25,139	4,596
Prepaid expenses and other current assets	10,822	9,391
Restricted cash	950	950
Total current assets	710,961	203,615
Property and equipment, net	15,044	12,539
Right-of-use assets, net	75,650	10,400
Goodwill	21,359	21,359
Intangible assets, net	81,280	85,536
Other assets	4,334	2,752
Total assets	908,628	336,201
Current liabilities:
Accounts payable	6,815	15,968
Accrued expenses	11,766	7,072
Current portion of deferred revenue	93,164	18,100
Current portion of operating lease liability	12,279	530
Total current liabilities	124,024	41,670
Contingent consideration	123,740	103,655
Deferred revenue, net of current portion	242,047	25,256
Operating lease liability, net of current portion	53,151	12,084
Total liabilities	542,962	182,665
Commitments and contingencies (Notes 3 and 12)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of September 30, 2020 and December 31, 2019; no shares issued and outstanding as of September 30, 2020 and December 31, 2019
Common stock, $0.001 par value; 200,000,000 shares authorized as of September 30, 2020 and December 31, 2019; 74,251,559 shares and 60,022,067 shares issued and outstanding as of September 30, 2020 and December 31, 2019, respectively	74	60
Additional paid-in capital	758,314	512,231
Accumulated deficit	(392,722)	(359,496)
Accumulated other comprehensive income		741
Total stockholders' equity	365,666	153,536
Total liabilities and stockholders' equity	$ 908,628	$ 336,201

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares Issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	74,251,559	60,022,067
Common stock, shares outstanding	74,251,559	60,022,067

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 66,446	$ 1,266	$ 87,420	$ 3,914
Operating expenses:
Research and development	26,344	17,295	76,785	51,343
General and administrative	9,163	6,881	25,223	21,284
Change in fair value of contingent consideration	14,190	(19,834)	20,085	(3,243)
Impairment of intangible asset		18,559	0	18,559
Total operating expenses	49,697	22,901	122,093	87,943
Income (loss) from operations	16,749	(21,635)	(34,673)	(84,029)
Other income, net	595	408	1,447	1,286
Income (loss) before benefit from income taxes	17,344	(21,227)	(33,226)	(82,743)
Benefit from income taxes				486
Net income (loss)	$ 17,344	$ (21,227)	$ (33,226)	$ (82,257)
Net income (loss) per share—basic	$ 0.24	$ (0.41)	$ (0.51)	$ (1.69)
Weighted average common shares outstanding—basic	73,183,923	51,891,157	65,187,435	48,574,275
Net income (loss) per share—diluted	$ 0.23	$ (0.41)	$ (0.51)	$ (1.69)
Weighted average common shares outstanding—diluted	76,440,293	51,891,157	65,187,435	48,574,275

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 17,344	$ (21,227)	$ (33,226)	$ (82,257)
Other comprehensive income (loss):
Unrealized gains (losses) on available-for-sale securities, net of tax of $0	(540)	109	(741)	483
Comprehensive income (loss)	$ 16,804	$ (21,118)	$ (33,967)	$ (81,774)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Unrealized gains on available-for-sale securities, net of tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Statem_4

Condensed Consolidated Statements Of Stockholders' Equity - USD ($) $ in Thousands	Total	Second Sanofi Amendment Agreement [Member]	Common Stock [Member]	Common Stock [Member]Second Sanofi Amendment Agreement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Second Sanofi Amendment Agreement [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balances at Dec. 31, 2018	$ 125,295		$ 45		$ 371,257		$ (246,203)	$ 196
Beginning balance, Shares at Dec. 31, 2018			45,139,955
Exercise of stock options	897				897
Exercise of stock options, Shares			154,484
Stock-based compensation expense	1,959				1,959
Unrealized gains (losses) on available-for-sale securities	155							155
Net income (loss)	(33,198)						(33,198)
Ending balances at Mar. 31, 2019	95,108		$ 45		374,113		(279,401)	351
Ending balance, shares at Mar. 31, 2019			45,294,439
Beginning balances at Dec. 31, 2018	125,295		$ 45		371,257		(246,203)	196
Beginning balance, Shares at Dec. 31, 2018			45,139,955
Issuance of common stock in connection with a former employee letter agreement	847
Unrealized gains (losses) on available-for-sale securities	483
Net income (loss)	(82,257)
Ending balances at Sep. 30, 2019	181,573		$ 60		509,294		(328,460)	679
Ending balance, shares at Sep. 30, 2019			60,020,725
Beginning balances at Mar. 31, 2019	95,108		$ 45		374,113		(279,401)	351
Beginning balance, Shares at Mar. 31, 2019			45,294,439
Issuance of common stock in connection with private placement, net of placement agent fees and offering costs	44,134		$ 6		44,128
Issuance of common stock in connection with private placement, net of placement agent fees and offering costs,shares			5,582,940
Issuance of common stock in connection with a former employee letter agreement	847				847
Issuance of common stock in connection with a former employee letter agreement , Shares			67,406
Forfeited restricted common stock	(1)				(1)
Forfeited restricted common stock, Shares			(1,334)
Exercise of stock options	519				519
Exercise of stock options, Shares			66,917
Stock-based compensation expense	2,703				2,703
Unrealized gains (losses) on available-for-sale securities	219							219
Net income (loss)	(27,832)						(27,832)
Ending balances at Jun. 30, 2019	115,697		$ 51		422,309		(307,233)	570
Ending balance, shares at Jun. 30, 2019			51,010,368
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs	84,011		$ 9		84,002
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs ,Shares			9,000,000
Forfeited restricted common stock, Shares			(449)
Exercise of stock options	84				84
Exercise of stock options, Shares			10,806
Stock-based compensation expense	2,899				2,899
Unrealized gains (losses) on available-for-sale securities	109							109
Net income (loss)	(21,227)						(21,227)
Ending balances at Sep. 30, 2019	181,573		$ 60		509,294		(328,460)	679
Ending balance, shares at Sep. 30, 2019			60,020,725
Beginning balances at Dec. 31, 2019	153,536		$ 60		512,231		(359,496)	741
Beginning balance, Shares at Dec. 31, 2019			60,022,067
Exercise of stock options	132				132
Exercise of stock options, Shares			15,596
Stock-based compensation expense	3,172				3,172
Unrealized gains (losses) on available-for-sale securities	114							114
Net income (loss)	(14,282)						(14,282)
Ending balances at Mar. 31, 2020	142,672		$ 60		515,535		(373,778)	855
Ending balance, shares at Mar. 31, 2020			60,037,663
Beginning balances at Dec. 31, 2019	153,536		$ 60		512,231		(359,496)	741
Beginning balance, Shares at Dec. 31, 2019			60,022,067
Ending balances at Jun. 30, 2020	271,393		$ 69		680,850		(410,066)	540
Ending balance, shares at Jun. 30, 2020			69,359,509
Beginning balances at Dec. 31, 2019	153,536		$ 60		512,231		(359,496)	741
Beginning balance, Shares at Dec. 31, 2019			60,022,067
Unrealized gains (losses) on available-for-sale securities	(741)
Net income (loss)	(33,226)
Ending balances at Sep. 30, 2020	365,666		$ 74		758,314		(392,722)
Ending balance, shares at Sep. 30, 2020			74,251,559
Beginning balances at Mar. 31, 2020	142,672		$ 60		515,535		(373,778)	855
Beginning balance, Shares at Mar. 31, 2020			60,037,663
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs	153,611		$ 9		153,602
Issuance of common stock in connection with public offering, net of underwriting discounts and commissions and offering costs ,Shares			8,544,982
Exercise of stock options	5,699				5,699
Exercise of stock options, Shares			776,864
Stock-based compensation expense	6,014				6,014
Unrealized gains (losses) on available-for-sale securities	(315)							(315)
Net income (loss)	(36,288)						(36,288)
Ending balances at Jun. 30, 2020	271,393		$ 69		680,850		(410,066)	540
Ending balance, shares at Jun. 30, 2020			69,359,509
Issuance of common stock in connection with Securities Purchase Agreement (Value)		$ 73,754		$ 5		$ 73,749
Issuance of common stock in connection with Securities Purchase Agreement (Shares)				4,884,434
Exercise of stock options	60				60
Exercise of stock options, Shares			7,616
Stock-based compensation expense	3,655				3,655
Unrealized gains (losses) on available-for-sale securities	(540)							$ (540)
Net income (loss)	17,344						17,344
Ending balances at Sep. 30, 2020	$ 365,666		$ 74		$ 758,314		$ (392,722)
Ending balance, shares at Sep. 30, 2020			74,251,559

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash flows - USD ($) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ 17,344	$ (14,282)	$ (21,227)	$ (33,198)	$ (33,226)	$ (82,257)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization expense					6,374	2,951
Stock-based compensation expense					12,841	8,408
Impairment of intangible asset			18,559		0	18,559	$ 18,559
Change in fair value of contingent consideration	14,190		(19,834)		20,085	(3,243)
Deferred income tax benefit						(486)
Changes in operating assets and liabilities:
Collaboration receivables					(20,543)	125
Prepaid expenses and other assets					(5,766)	(2,293)
Right-of-use assets					1,338	357
Long-term prepaid rent					(10,057)	(2,492)
Accounts payable					(8,560)	(1,740)
Accrued expenses					4,913	2,244
Lease liability					(962)	(270)
Deferred revenue					291,855	(1,763)
Net cash provided by (used in) operating activities					258,292	(61,900)
Cash flows from investing activities:
Purchases of investments					(27,409)	(138,156)
Sales and maturities of investments					130,765	116,311
Purchases of property and equipment					(5,434)	(2,241)
Net cash provided by (used in) investing activities					97,922	(24,086)
Cash flows from financing activities:
Proceeds from public offerings, net of underwriting discounts and commissions					154,292	84,600
Payments of public offering costs					(681)	(589)
Proceeds from Securities Purchase Agreement					73,754
Proceeds from private placement, net of placement agent fees						44,608
Payments of private placement offering costs						(477)
Proceeds from option exercises					5,891	1,500
Net cash provided by financing activities					233,256	129,642
Net increase in cash, cash equivalents and restricted cash					589,470	43,656
Cash, cash equivalents and restricted cash at beginning of period		85,530		56,224	85,530	56,224	56,224
Cash, cash equivalents and restricted cash at end of period	675,000		99,880		675,000	99,880	85,530
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	674,050		98,930		674,050	98,930	84,580
Restricted cash	950		950		950	950	950
Total cash, cash equivalents and restricted cash at end of period	$ 675,000	$ 85,530	$ 99,880	$ 56,224	675,000	99,880	$ 56,224
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses					$ 377	2
Issuance of common stock in connection with a former employee letter agreement						$ 847

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Translate Bio, Inc. (the “Company”) is a clinical-stage messenger RNA (“mRNA”) therapeutics company developing a new class of potentially transformative medicines to treat diseases caused by protein or gene dysfunction, or to prevent infectious diseases by generating protective immunity. Using its proprietary mRNA therapeutic platform (“MRT platform”), the Company creates mRNA that encodes functional proteins. The Company’s mRNA is designed to be delivered to the target cell where the cell’s own machinery recognizes it and translates it, restoring or augmenting protein function to treat or prevent disease. The Company is primarily focused on applying its MRT platform to treat pulmonary diseases caused by insufficient protein production or where production of proteins can modify disease. The Company is also pursuing the applicability of its MRT platform for the development of mRNA vaccines for infectious diseases under a collaboration with Sanofi Pasteur Inc. (“Sanofi”), the vaccines global business unit of Sanofi S.A. The outbreak of a novel strain of coronavirus named SARS-CoV-2 (“COVID-19”), COVID-19 pandemic. COVID-19 COVID-19, COVID-19 The Company is developing MRT5005 for the treatment of CF. The Company is conducting a Phase 1/2 clinical trial to evaluate the safety and tolerability of single and multiple-ascending doses of MRT5005. Percent predicted forced expiratory volume in one second (“ppFEV 1 pre-defined one-month mid-dose 1 mid-dose COVID-19 pandemic. COVID-19 The Company is leveraging its lung delivery platform and focusing its preclinical research efforts on identifying lead product candidates for a next-generation CF program, as well as beyond CF in additional pulmonary diseases with unmet medical need, including primary ciliary dyskinesia, idiopathic pulmonary fibrosis and pulmonary arterial hypertension. The Company has a collaboration with Sanofi to develop infectious disease vaccines using the Company’s mRNA technology. Under the collaboration, the Company and Sanofi will jointly conduct research and development activities to advance mRNA vaccines targeting up to seven infectious disease pathogens (see Note 3). Two of the target pathogens under development are SARS-CoV-2 SARS-CoV-2. The Company is subject to risks common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The preparation of the accompanying condensed consolidated financial statements requires the Company to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses and related disclosure of contingent assets and liabilities. On an ongoing basis the Company evaluates its estimates, judgments and methodologies. The Company bases its estimates on historical experience and on various other assumptions that it believes are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenues and expenses. The full extent to which the COVID-19 COVID-19 COVID-19, COVID-19 The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its two wholly owned subsidiaries, Translate Bio MA, Inc. and Translate Bio Securities Corporation, from their date of incorporation. All intercompany accounts and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated balance sheet as of September 30, 2020, the unaudited condensed consolidated statements of operations and of comprehensive income (loss) for the three and nine months ended September 30, 2020 and 2019, the unaudited condensed consolidated statements of stockholders’ equity for the three and nine months ended September 30, 2020 and 2019 and the unaudited condensed consolidated statements of cash flows for the nine months ended September 30, 2020 and 2019 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. The accompanying balance sheet as of December 31, 2019 has been derived from the Company’s audited financial statements for the year ended December 31, 2019. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. The accompanying unaudited interim condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K The accompanying unaudited interim condensed consolidated financial presentation has been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflects all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2020, the results of its operations for the three and nine months ended September 30, 2020 and 2019, and its cash flows for the nine months ended September 30, 2020 and 2019. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2020 and 2019 are also unaudited. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. Sales of Common Stock On March 13, 2020, the Company filed a universal shelf registration statement on Form S-3 No. 333-237159). The Company is party to an Open Market Sale Agreement SM As of September 30, 2020, the Company has issued and sold an aggregate of 2,863,163 shares of its common stock pursuant to the Sales Agreement, resulting in gross proceeds of $37.9 million, before deducting commissions of $1.1 million and other offering expenses of $0.2 million. There were no shares issued or sold pursuant to the Sales Agreement during the three months ended September 30, 2020. In the future, $62.1 million of shares of common stock remain available to be sold pursuant to the Sales Agreement, which sales, if any, would be made under the 2020 Shelf. On June 24, 2020, the Company filed a registration statement on Form S-3ASR, No. 333-239405) Sanofi Pasteur Collaboration and Licensing Agreement In 2018, the Company entered into a collaboration and license agreement with Sanofi (the “Original Sanofi Agreement”) to develop mRNA vaccines for up to five infectious disease pathogens (the “Licensed Fields”). On March 26, 2020, the Company and Sanofi amended the Original Sanofi Agreement (the “First Sanofi Amendment”) to include vaccines against SARS-CoV-2 Pursuant to the Amended Sanofi Agreement, the Company and Sanofi will research collaboration expires in June 2022 with an option for Sanofi to extend for one additional year. If Sanofi elects to e the collaboration up to Under the terms of the Amended Sanofi Agreement, the Company has granted to Sanofi exclusive, worldwide licenses under applicable patents, patent applications, know-how Pursuant to the Second Sanofi Amendment, Sanofi pa id 20-day Liquidity In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40) The Company’s financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. Through September 30, 2020, the Company has funded its operations primarily through sales of equity securities and research and development collaboration agreements. The Company has incurred recurring losses and cash outflows from operations since its inception, including net losses of $33.2 million and $82.3 million for the nine months ended September 30, 2020 and 2019, respectively. In addition, the Company had an accumulated deficit of $392.7 million as of September 30, 2020. The Company expects to continue to generate operating losses for the foreseeable future. As of November 5, 2020, the date of issuance of these unaudited interim condensed consolidated financial statements, the Company expects that its cash and cash equivalents of $ million as of September , will be sufficient to fund its operating expenses and capital expenditure requirements through 2023 . The future viability of the Company beyond that point is dependent on the Company’s ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. The Company expects that its expenses will increase in connection with its ongoing business activities. As a result, the Company will need substantial additional funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of equity, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may be unable to raise additional funds or enter into such other agreements or arrangements when needed on favorable terms, or at all. If the Company is unable to obtain funding, the Company will be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The significant accounting policies and estimates used in preparation of the consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. Recently Adopted Accounting Pronouncements In January 2017, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2017-04, Intangibles—Goodwill and Other: Simplifying the Test for Goodwill Impairment (Topic 350) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. Revenue from Contracts with Customers Collaborative Arrangements Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments available-for-sale In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new guidance also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Sanofi Collaboration and License Agreement	3. Collaboration Agreement Sanofi Collaboration and License Agreement In 2018, the Company and Sanofi entered into the Original Sanofi Agreement to develop mRNA vaccines and an mRNA vaccine platform for up to five infectious disease pathogens. In March 2020, the Company and Sanofi entered into the First Sanofi Amendment to include vaccines against SARS-CoV-2 In June 2020, the Company and Sanofi entered into the Second Sanofi Amendment, which became effective in July 2020 following early termination of the waiting period under the Hart-Scott Rodino Act, to expand the scope of the collaboration and licenses granted to Sanofi. Under the terms of the Amended Sanofi Agreement, the Company has agreed to grant to Sanofi exclusive, worldwide licenses under applicable patents, patent applications, know-how . Pursuant to the Amended Sanofi Agreement, the Company and Sanofi will jointly conduct research and development activities to advance mRNA vaccines targeting up to seven infectious disease pathogens. The term of the research collaboration (the “Collaboration Term”) expires in June 2022, with an option for Sanofi to extend the Collaboration Term for one additional year, followed by a technology transfer to Sanofi. If Sanofi elects to extend the Collaboration Term, the collaboration may be further expanded to jointly conduct research and development activities to advance mRNA vaccines for up to an additional three infectious disease pathogens, bringing the total to up to ten pathogens. In addition to the research and development and technology transfer, the Company is responsible for manufacturing and supplying non-clinical out-of-pocket The Company and Sanofi non-exclusive, Pursuant to the Original Sanofi Agreement Sanofi paid id payment of SARS-CoV-2 SARS-CoV-2 one-time Under the terms of the Amended Sanofi Agreement, Sanofi has also agreed to pay the Company royalties on net sales of mRNA vaccines in the SARS-CoV-2 The Amended Sanofi Agreement provides that it will remain in effect until terminated in accordance with its terms. Either the Company or Sanofi may terminate the Amended Sanofi Agreement in its entirety if the other party is subject to certain insolvency proceedings. Either party may terminate the Amended Sanofi Agreement in its entirety or with respect to a particular Licensed Field, country or product if the other party materially breaches the Amended Sanofi Agreement and the breach remains uncured for a specified period, which may be extended in certain circumstances. Sanofi may also terminate the Amended Sanofi Agreement in its entirety or with respect to a particular Licensed Field, country or product for safety reasons or for convenience, in each case after a specified notice period. After termination of the Amended Sanofi Agreement, Sanofi may continue to manufacture and commercialize the terminated products for a specified period of time, subject to Sanofi’s payment obligations. In connection with the execution of the Second Sanofi Amendment, the Company and the Investor entered into the Securities Purchase Agreement for the sale and issuance of 4,884,434 shares of the Company’s common stock to the Investor at a price of $25.59 per share representing a 50 percent premium to the 20-day in “Lock-Up Lock-Up Sanofi has sole responsibility for all commercialization activities for mRNA vaccines in the Licensed Fields and is obligated to bear all costs in connection with any commercialization in the Licensed Fields. The Company and Sanofi also entered into a separate supply agreement on June 22, 2020, with an effective date of December 20, 2019, governing the terms of the supply of products by the Company (the “Supply Agreement”). Pursuant to the Supply Agreement, the Company has agreed to use commercially reasonable efforts to manufacture and supply Sanofi with non-clinical non-clinical SARS-CoV-2 Accounting for the Sanofi Collaboration For accounting purposes, the Company has combined the Amended Sanofi Agreement, Securities Purchase Agreement, and Supply Agreement because the contracts were negotiated as a package with a single commercial objective, the amount of consideration to be paid in one contract depends on the price or performance of the other contracts, and the goods and services promised in the contracts are a single performance obligation in accordance with ASC 606. The Company accounts for the Amended Sanofi Agreement under ASC 606. In determining the appropriate amount of revenue to be recognized under ASC 606, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company identified the following promised goods or services contained in the Amended Sanofi Agreement: (i) the license it conveyed to Sanofi with respect to the Licensed Fields, (ii) the licensed know-how non-clinical services represent one performance obligation. Additionally, Sanofi’s right to exercise its option to extend the research term and have the Company conduct research and development activities to advance mRNA vaccines for up to an additional three infectious disease pathogens were determined at the time of the modification to not represent material rights, based on the criteria of ASC 606, and therefore do not represent a separate performance obligation. Under ASC 606, the Company recognized revenue using the cost-to-cost cost-to-cost The Company has accounted for the Second Sanofi Amendment as a modification to the existing agreement and not as a separate agreement because the additional goods and services are not distinct and therefore form a single performance obligation that is partially satisfied at the date of the contract modification. The effect that the modification has on the transaction price, and on the Company’s measure of progress toward complete satisfaction of the performance obligation, was recognized as a cumulative catch-up During the three and nine months ended September 30, 2020, the Company increased the overall transaction price by $491.8 million and $534.7 million, respectively. The transaction price as of September 30, 2020 includes the upfront, non-refundable , which represents the excess of the price paid compared to the fair value of the Company’s common stock on the closing date, under the Securities Purchase Agreement, an estimated $71.8 million in reimbursable employee costs, an estimated $155.7 million in reimbursable development costs including manufacturing costs and out-of-pocket re-evaluates re-evaluates re-evaluated re-evaluate The following table summarizes the Company’s collaboration revenue (in thousands): Three Months Ended Nine Months Ended 2020 2019 2020 2019 Collaboration revenue $ 66,446 $ 1,266 $ 87,420 $ 3,914 The following table presents the balance of the Company’s contract liabilities (in thousands): September 30, 2020 December 31, 2019 Contract liabilities Deferred revenue $ 335,211 $ 43,356 Deferred revenue is classified as short-term or long-term in the consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next twelve months which is determined by the cost-to-cost input method which measures the extent of progress towards satisfying the performance obligation. As of September 30, 2020, the aggregate amount of the transaction price allocated to the remaining performance obligation is estimated to be approximately $589.2 million, which is expected to be recognized as revenue through 2024. Revenue recognized from contract liabilities at the beginning of the period was $3.5 million and $1.8 million during the nine months ended September 30, 2020 and 2019, respectively.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	9 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	4. Intangible Assets and Goodwill Acquisition of Shire’s MRT Program In December 2016, the Company entered into an asset purchase agreement (as amended in June 2018) with Shire Human Genetic Therapies, Inc. (“Shire”), a subsidiary of Takeda Pharmaceutical Company Ltd., pursuant to which Shire sold equipment to and assigned to the Company all of its rights to certain patent rights, permits, real property leases, contracts, regulatory documentation, books and records, and materials related to Shire’s mRNA therapy platform (the “MRT Program”), including its cystic fibrosis transmembrane conductance regulator program. Intangible Assets, Net The acquisition of Shire’s MRT Program was accounted for in accordance with the acquisition method of accounting for business combinations. The total purchase consideration transferred was allocated to the tangible and identifiable intangible assets acquired based on their estimated fair values. The tables below present the Company’s definite-lived intangible assets that are subject to amortization and indefinite-lived intangible assets: September 30, 2020 Estimated Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 6 years $ 45,992 $ (7,003 ) $ — $ 38,989 Indefinite-lived intangible assets: IPR&D - CF Indefinite 42,291 — — 42,291 Total intangible assets, net $ 88,283 $ (7,003 ) $ — $ 81,280 December 31, 2019 Estimated Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (2,747 ) $ — $ 43,245 Indefinite-lived intangible assets: IPR&D - CF Indefinite 42,291 — — 42,291 IPR&D - OTC Indefinite 18,559 — (18,559 ) — Total intangible assets, net $ 106,842 $ (2,747 ) $ (18,559 ) $ 85,536 Identifiable intangible assets acquired in the acquisition of Shire’s MRT Program consisted of in-process Upon commencement of the Original Sanofi Agreement, the IPR&D - $11.3 million , Indefinite-lived IPR&D is not subject to amortization, but is tested annually for impairment or more frequently if there are indicators of impairment. The Company tests its indefinite-lived IPR&D annually for impairment on October 1 st , no Goodwill The excess of the fair value of the consideration transferred over the fair value of identifiable assets acquired in the acquisition of Shire’s MRT Program was allocated to goodwill in the amount of $21.4 million. There have been no changes to the carrying amount of goodwill during the nine months ended September 30, 2020. Goodwill is not subject to amortization, but is tested annually for impairment or more frequently if there are indicators of impairment. The Company tests its goodwill annually for impairment on October 1 st

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	5. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis (in thousands): Fair Value Measurements as of September 30, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 636,615 $ — $ 636,615 U.S. government agency bonds — — — — $ — $ 636,615 $ — $ 636,615 Liabilities: Contingent consideration $ — $ — $ 123,740 $ 123,740 $ — $ — $ 123,740 $ 123,740 Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 56,591 $ — $ 56,591 U.S. government agency bonds — 104,098 — 104,098 $ — $ 160,689 $ — $ 160,689 Liabilities: Contingent consideration $ — $ — $ 103,655 $ 103,655 $ — $ — $ 103,655 $ 103,655 During the nine months ended September 30, 2020 and the year ended December 31, 2019, there were no transfers between Level 1, Level Cash equivalents as of September available-for-sale Valuation of Contingent Consideration The contingent consideration liability related to the acquisition of Shire’s MRT Program in 2016 was classified as a Level 3 measurement The fair value of the liability to make potential future milestone and earnout payments was estimated by the Company at each reporting date based, in part, on the results of a third-party valuation using a discounted cash flow analysis based on various assumptions, including the probability of achieving specified events, discount rates, and the period of time until earnout payments are payable and the conditions triggering the milestone payments are met. The actual settlement of contingent consideration could differ from current estimates based on the actual occurrence of these specified events. The following table presents the unobservable inputs and fair value of the components of the contingent consideration (dollar amounts in thousands): Unobservable Fair Value at Projected Year of September 30, 2020 December 31, 2019 Earnout payments 2026 - 115,079 $ 96,097 Milestone payments 2026 - 8,661 7,558 $ 123,740 $ 103,655 The discount rate used in the third-party valuation was 12.5% and 13.5% as of September 30, 2020 and December 31, 2019, respectively. The following table presents a roll-forward of the total acquisition-related contingent consideration liability (in thousands): Fair Value Balance as of December 31, 2019 $ 103,655 Increase in fair value of contingent consideration 20,085 Balance as of September 30, 2020 $ 123,740 The increase in the fair value of contingent consideration was primarily due to the time value of money due to the passage of time and a decrease in the discount rate.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	6. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): September 30, 2020 December 31, 2019 Laboratory equipment $ 11,763 $ 9,044 Computer equipment 892 779 Office equipment 942 883 Leasehold improvements 5,730 5,635 Construction in progress 5,094 3,460 24,421 19,801 Less: Accumulated depreciation and amortization (9,377 ) (7,262 ) $ 15,044 $ 12,539 Depreciation and amortization expense related to property and equipment was $0.8 million and $0.6 million during the three months ended September 30, 2020 and 2019, respectively, and $2.1 million and $1.8 million during the nine months ended September 30, 2020 and 2019, respectively.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2020
Payables and Accruals [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following (in thousands): September 30, December 31, Accrued employee compensation and benefits $ 4,918 $ 3,547 Accrued external research and development expenses 2,058 1,763 Accrued consultant and professional fees 1,358 1,390 Other 3,432 372 $ 11,766 $ 7,072 Included in other accrued expenses as of September 30, 2020 are costs related to entering into the Second Sanofi Agreement.

Incentive Stock Options and Res

Incentive Stock Options and Restricted Stock	9 Months Ended
	Sep. 30, 2020
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Incentive Stock Options and Restricted Stock	8. Incentive Stock Options and Restricted Stock 2018 Equity Incentive Plan On March 7, 2018, the Company’s non-qualified The number of shares initially reserved for issuance under the 2018 Plan is the sum of 2,512,187, plus the number of shares (up to 1,013,167 shares) equal to the sum of (i) the number of shares remaining available for issuance under the 2016 Stock Incentive Plan, as amended (the “2016 Plan”), upon the effectiveness of the 2018 Plan, which was 360,514 shares, and (ii) the number of shares of common stock subject to outstanding awards under the 2016 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right. The number of shares of common stock that may be issued under the 2018 Plan will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2019 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2028, by an amount equal to the lowest of (i) 3,349,582 shares, (ii) 4% of the outstanding shares of common stock on such date and (iii) an amount determined by the Board of Directors. As of December 31, 2019, there were 4,829,847 shares of common stock reserved for issuance under the 2018 Plan. On January 1, 2020, the number of shares of common stock that may be issued under the 2018 Plan increased by 2,400,829 shares of common stock. During the nine months ended September 30, 2020, a total of 99,329 shares issued under the 2016 Plan have been cancelled and rolled over to the 2018 Plan, such that there is a total of 7,330,005 shares of common stock reserved for issuance under the 2018 Plan as of September 30, 2020. The shares of common stock underlying any awards that are forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2018 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. The 2018 Plan is administered by the Board of Directors. The exercise prices, vesting periods and other restrictions are determined at the discretion of the Board of Directors, except that the exercise price per share of options may not be less than 100% of the fair market value of the common stock on the date of grant. Stock options awarded under the 2018 Plan expire 10 years after the grant date, unless the Board of Directors sets a shorter term. Awards granted to employees, officers, members of the Board of Directors and consultants typically vest over a period of one Typically, unvested stock options are forfeited upon the recipient ceasing to provide services to the Company. 2018 Employee Stock Purchase Plan On March 7, 2018, the Board of Directors, subject to stockholder approval, adopted, and on June 15, 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the “2018 ESPP”), which became effective on June 27, 2018. A total of 418,697 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2018 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2019 and continuing for each fiscal year until, and including, the fiscal year commencing on January 1, 2029, by an amount equal to the lowest of (i) 837,395 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Board of Directors. In December 2019, the Board of Directors elected to add no shares of common stock to the 2018 ESPP. As of September 30, 2020, 870,096 shares of common stock were reserved for issuance under this plan. 2016 Stock Incentive Plan The 2016 Plan provided for the grant of stock options, stock appreciation rights, restricted stock and restricted stock units. Shares that are expired, terminated, surrendered or canceled under the 2016 Plan without having been exercised will be available for future grants of awards under the 2018 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2018 Plan. The 2016 Plan is administered by the Board of Directors. The exercise prices, vesting periods and other restrictions were determined at the discretion of the Board of Directors, except that the exercise price per share of options could not be less than 100% of the fair market value of the common stock on the date of grant. Stock options awarded under the 2016 Plan expire 10 years after the grant date, unless the Board of Directors set a shorter term. Stock options and restricted stock granted to employees, officers, members of the Board of Directors and consultants typically vest over a four-year period. Upon the effectiveness of the 2018 Plan on June 27, 2018, no further awards will be made under the 2016 Plan, but awards outstanding under the 2016 Plan will continue to be governed by their existing terms. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2019 (in thousands, except share and per share amounts): Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Intrinsic Value (in years) Outstanding as of December 31, 2019 8,646,378 $ 8.06 8.42 $ 3,687 Granted 2,992,734 $ 9.22 Exercised (800,076 ) $ 7.36 Forfeited (529,492 ) $ 8.59 Outstanding as of September 30, 2020 10,309,544 $ 8.42 8.04 $ 55,026 Exercisable as of September 30, 2020 4,915,617 $ 7.96 7.23 $ 27,795 Vested and expected to vest as of September 30, 2020 10,309,544 $ 8.42 8.04 $ 55,026 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2020 and 2019 was $10.7 million and $0.6 million, respectively. The weighted average grant-date fair value per share of stock options granted was $5.65 and $5.58 during the nine months ended September 30, 2020 and 2019, respectively. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company completed its initial public offering in July 2018 and therefore lacks company-specific historical and implied volatility information before that date. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees and directors: Nine Months Ended 2020 2019 Risk-free interest rate 0.77 % 2.39 % Expected term (in years) 6.1 6.0 Expected volatility 68.7 % 73.1 % Expected dividend yield 0 % 0 % Restricted Common Stock The following table summarizes the Company’s restricted stock activity since December 31, 2019: Number of Shares Weighted Average Grant-Date Fair Value Unvested restricted common stock outstanding as of December 31, 2019 34,168 $ 1.28 Forfeited restricted common stock — $ — Vested restricted common stock (34,168 ) $ 1.28 Unvested restricted common stock outstanding as of September 30, 2020 — $ — Stock-Based Compensation Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows (in thousands): Three Months Ended Nine Months Ended 2020 2019 2020 2019 Research and development expenses $ 1,578 $ 1,301 $ 7,123 $ 3,454 General and administrative expenses 2,077 1,598 5,718 4,954 $ 3,655 $ 2,899 $ 12,841 $ 8,408 Included in research and development stock-based compensation expense for the nine months ended September 30, 2020 was $2.4 million related to the modification of options in connection with the resignation of the Company’s former Executive Vice President and Founder (“EVP and Founder”). In connection with this resignation the Company entered into a separation agreement with the EVP and Founder. Under the terms of the separation agreement, vesting of options for the purchase of 176,266 shares of common stock held by the EVP and Founder was accelerated with no change to the exercise price of such options. Stock options for the purchase of 550,278 shares of common stock, representing all of the options held by the EVP and Founder as of the date of his resignation, are exercisable for 18 months following his resignation. As of September 30, 2020, total unrecognized compensation

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company recognized an income tax benefit of $0 and $0.5 million

Net Income (Loss) per Share

Net Income (Loss) per Share	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Income (Loss) per Share	10. Net Income (Loss) per Share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common share equivalents outstanding for the period, including any dilutive effect from outstanding stock options. Basic and diluted net income (loss) per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended Nine Months Ended 2020 2019 2020 2019 Basic net income (loss) per common share: Numerator: Net income (loss) $ 17,344 $ (21,227 ) $ (33,226 ) $ (82,257 ) Denominator: Weighted average common shares 73,183,923 51,891,157 65,187,435 48,574,275 Net income (loss) per share—basic $ 0.24 $ (0.41 ) $ (0.51 ) $ (1.69 ) Diluted net income (loss) per common share: Numerator: Net income (loss) $ 17,344 $ (21,227 ) $ (33,226 ) $ (82,257 ) Denominator: Weighted average common shares 76,440,293 51,891,157 65,187,435 48,574,275 Net income (loss) per share—diluted $ 0.23 $ (0.41 ) $ (0.51 ) $ (1.69 ) The Company excluded 99,468 and 147,842 shares of restricted common stock, presented on a The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Options to purchase common stock — 8,568,506 10,309,544 8,568,506 Unvested restricted common stock — 72,677 — 72,677 — 8,641,183 10,309,544 8,641,183

Leases

Leases	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Leases	11. Leases Suite Retention and Development Agreement In September 2019, the Company entered into a suite retention and development agreement with Albany Molecular Research, Inc. (“AMRI”) under which a series of cleanroom suites were built at AMRI’s manufacturing facility in accordance with the Company’s objectives (“AMRI Agreement”). The AMRI Agreement continues for five years build-out The Company has determined this is a lease under ASU No. 2016-02, Under the AMRI Agreement, the Company will finance $6.0 million of the costs of the build-out (“Build-Out Build-Out Build-Out addit ional Build-Out right-of-use build-out build-out Real Estate Lease In June 2017, the Company entered into an operating lease for office and laboratory space at its headquarters in Lexington, Massachusetts 10-year Equipment Lease In March 2018, the Company entered into an operating lease for communications equipment for use at its office and laboratory space in Lexington, Massachusetts. The term of the lease is five years, expiring in March 2023 The Company excludes leases with an initial term of one year or less in the recognized ROU asset and lease liabilities. The Company recognizes lease expense for these leases on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, lease and non-lease components are combined into a single lease component. The Company’s leases have remaining lease terms of up to , excluding two five-year options to extend the real estate lease after the expiration of the initial term. The Company believes the real estate lease for office and laboratory spaces will be sufficient to meet its needs for the foreseeable future and that suitable additional space will be available as and when needed. The components of lease cost were as follows (dollar amounts in thousands): Nine Months Ended 2020 2019 Lease cost Operating lease cost $ 3,397 $ 2,019 Total lease cost $ 3,397 $ 2,019 Other information Operating cash flows from operating leases $ 3,022 $ 1,933 Operating lease liabilities arising from obtaining right-of-use $ 53,778 — Weighted-average remaining lease term 5 years 8 years Weighted-average discount rate 11.9 % 17.5 % As of September 30, 2020, maturities of operating lease liabilities are as follows (in thousands): September 30, 2020 $ 7,784 2021 14,737 2022 15,178 2023 15,591 2024 16,050 2025 and thereafter 19,164 Total future minimum lease payments 88,504 Less: imputed interest (23,074 ) Present value of lease liabilities $ 65,430 As of December 31, 2019, minimum rental commitments under the real estate lease was as follows (in thousands): December 31, 2020 $ 2,659 2021 2,737 2022 2,818 2023 2,860 2024 2,937 2025 and thereafter 10,160 Total future minimum lease payments 24,171 Less: imputed interest (11,557 ) Present value of lease liabilities $ 12,614 As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate, which are the rates incurred to borrow on a collateralized basis over a term equal to the lease payments in a similar economic environment, in determining the present value of lease payments. The Company used the incremental borrowing rate on January 1, 2019 for operating leases that commenced prior to that date and for all subsequent leases the Company used an appropriate borrowing rate upon commencement date.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Research, Supply and License Agreements Roche Master Supply Agreement The Company is a party to a master supply agreement with Roche Diagnostics Corporation (“Roche”) pursuant to which Roche will custom manufacture certain products for the Company. The agreement requires the Company to purchase from Roche specified manufactured products and the related raw materials in an amount equal to the greater of (i) quantities of raw materials in the Company’s annual forecast to be purchased or (ii) 80% of the Company’s demand for products as the same or similar type (the “Purchase Commitment”). In June 2017, the Company exercised its option under the agreement to extend the agreement through December 31, 2024. In September 2018, the Company and Roche amended the agreement to remove and replace the Purchase Commitment for certain manufactured products and related raw materials supplied by Roche. The agreement, as amended, specifies a minimum purchase requirement for certain custom manufactured products. As of September 30, 2020, the Company’s purchase commitments under the agreement totaled $14.0 million, with $3.5 million committed as payments each year from 2021 to 2024. Research and development expenses related to this agreement totaled less than $0.1 million and $1.8 million during the three months ended September 30, 2020 and 2019, respectively, and $2.7 million and $5.4 million during the nine months ended September 30, 2020 and 2019, respectively. MIT Research Agreement In September 2019, the Company entered into a research agreement with the Massachusetts Institute of Technology (“MIT”) pursuant to which the Company is obligated to reimburse MIT up to $4.1 million for specified direct and indirect costs to be incurred from January 2020 through December 2022 for specified research activities conducted for the Company (the “2019 MIT Agreement”). As of September 30, 2020, the Company paid MIT $1.4 million towards the total committed amount. Research and development expenses related to this agreement during the three and nine months ended September 30, 2020 were $0.3 million and $1.0 million, respectively. There were no amounts payable by the Company under the agreement as of September 30, 2020. The 2019 MIT Agreement expires in December 2022 and may be extended thereafter by mutual agreement of the parties. MIT Exclusive Patent License Agreement The Company is a party to at least one exclusive patent license agreement with MIT pursuant to which the Company received an exclusive license under the licensed patent rights to develop, manufacture and commercialize any product containing both (i) any RNA sequences, including mRNA, that encode a protein or peptide suitable for human therapeutic use which may include operably linked non-coding non-coding The Company has the right to grant sublicenses under this license. The patent rights licensed to the Company by MIT include claims that cover certain of the Company’s customized lipid-based nanoparticles used for delivery of coding RNA components in its MRT platform, including products that may be developed under the Company’s collaboration with Sanofi. Under the license agreement, the Company is obligated to make annual license maintenance payments to MIT, payable on January 1 of each calendar year, of up to $0.2 million, which may be credited against royalties subsequently due on net sales of licensed products earned in the same calendar year. The Company paid no annual license maintenance fees to MIT during each of the three months ended September 30, 2020 and 2019 and paid $0.2 million during each of the nine months ended September 30, 2020 and 2019. The Company is also obligated to make milestone payments to MIT aggregating up to $1.375 million upon the achievement of specified clinical and regulatory milestones with respect to each licensed product and $1.250 million upon the Company’s first commercial sale of each licensed product, and to pay royalties of a low single-digit percentage to MIT based on the Company’s, and any of its affiliates’ and sublicensees’, net sales of licensed products. The royalties are payable on a product-by-product country-by-country last-to-expire MIT-licensed The amount payable to MIT is currently being negotiated between the Company and MIT and the final amount payable to MIT is unknown as this time. The agreement obligates the Company to use commercially reasonable efforts and expend a minimum amount of resources each year to develop licensed products in accordance with a development plan, and a development milestone timetable specified in the agreement; to use commercially reasonable efforts to commercialize licensed products; and upon commercialization, to make the licensed products reasonably available to the public. MIT has the right to terminate the agreement if the Company fails to pay amounts when due or otherwise materially breaches the agreement and fails to cure such nonpayment or breach within specified cure periods or in the event the Company ceases to carry on its business related to the agreement. In the event of a termination due to the Company’s breach caused by a due diligence failure of a licensed product, but where the Company has fulfilled its obligations with respect to a different licensed product, MIT may not terminate the agreement with respect to the different licensed product. MIT may immediately terminate the agreement if the Company or any of its affiliates brings specified patent challenges against MIT or assists others in bringing a patent challenge against MIT. The Company has the right to terminate the agreement for its convenience at any time on three months’ prior written notice to MIT and payment of all amounts due to MIT through the date of termination. The Company’s patent rights, and the rights of its affiliates and sublicensees, in specified licensed products may also terminate, if the Company, its affiliates or MIT receives a request from a third party to develop such licensed product for which the Company is unable to, within nine months of receiving notice of any such request, either demonstrate that the Company has initiated a fully funded project for the commercial development of such licensed product, and provide a business plan with acceptable milestones; demonstrate that the licensed product proposed by such third party would be competitive with a licensed product for which the Company has initiated a fully funded project; or enter into a sublicense agreement with such third party on commercially reasonable terms, and, in each case, MIT, in its sole discretion, grants a license to such third party for the specified patent rights. E Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of September 30, 2020 and December 31, 2019. Legal Proceedings The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Related Party Transactions	13. Related Party Transactions Private Placement In connection with a private placement of the Company’s common stock in May 2019, entities affiliated with Baupost Group, L.L.C. (“Baupost”), a substantial stockholder, purchased 2,352,941 shares of the Company’s common stock at a price per share of $8.50 for an aggregate purchase price of $20.0 million. Public Offerings In connection with a public offering of the Company’s common stock in September 2019, Baupost purchased 5,000,000 shares of the Company’s common stock at a price per share of $10.00 for an aggregate purchase price of $50.0 million. In connection with a public offering of the Company’s common stock in June 2020, Baupost purchased 500,000 shares of the Company’s common stock at a price per share of $22.00 for an aggregate purchase price of $11.0 million.

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2020
Subsequent Events [Abstract]
Subsequent Event	14. Subsequent Event Real Estate Lease On November 3, 2020, (the “Lease Commencement Date”), the Company entered into a ten-year

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Recently Adopted Accounting Pronouncement	Recently Adopted Accounting Pronouncements In January 2017, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2017-04, Intangibles—Goodwill and Other: Simplifying the Test for Goodwill Impairment (Topic 350) In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. Revenue from Contracts with Customers Collaborative Arrangements
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments available-for-sale In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new guidance also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up

Collaboration Agreement (Tables

Collaboration Agreement (Tables)	9 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Collaboration Revenue	The following table summarizes the Company’s collaboration revenue (in thousands): Three Months Ended Nine Months Ended 2020 2019 2020 2019 Collaboration revenue $ 66,446 $ 1,266 $ 87,420 $ 3,914
Balance of Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of the Company’s contract liabilities (in thousands): September 30, 2020 December 31, 2019 Contract liabilities Deferred revenue $ 335,211 $ 43,356

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	9 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Definite-Lived Intangible Assets Subject to Amortization and Indefinite-Lived Intangible Assets	The tables below present the Company’s definite-lived intangible assets that are subject to amortization and indefinite-lived intangible assets: September 30, 2020 Estimated Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 6 years $ 45,992 $ (7,003 ) $ — $ 38,989 Indefinite-lived intangible assets: IPR&D - CF Indefinite 42,291 — — 42,291 Total intangible assets, net $ 88,283 $ (7,003 ) $ — $ 81,280 December 31, 2019 Estimated Gross Carrying Amount Accumulated Amortization Impairment Charge Net Carrying Amount (In thousands) Definite-lived intangible assets: MRT 8 years $ 45,992 $ (2,747 ) $ — $ 43,245 Indefinite-lived intangible assets: IPR&D - CF Indefinite 42,291 — — 42,291 IPR&D - OTC Indefinite 18,559 — (18,559 ) — Total intangible assets, net $ 106,842 $ (2,747 ) $ (18,559 ) $ 85,536

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis (in thousands): Fair Value Measurements as of September 30, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 636,615 $ — $ 636,615 U.S. government agency bonds — — — — $ — $ 636,615 $ — $ 636,615 Liabilities: Contingent consideration $ — $ — $ 123,740 $ 123,740 $ — $ — $ 123,740 $ 123,740 Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ — $ 56,591 $ — $ 56,591 U.S. government agency bonds — 104,098 — 104,098 $ — $ 160,689 $ — $ 160,689 Liabilities: Contingent consideration $ — $ — $ 103,655 $ 103,655 $ — $ — $ 103,655 $ 103,655
Schedule of Unobservable Inputs and Fair Value Components of Contingent Consideration	The following table presents the unobservable inputs and fair value of the components of the contingent consideration (dollar amounts in thousands): Unobservable Fair Value at Projected Year of September 30, 2020 December 31, 2019 Earnout payments 2026 - 115,079 $ 96,097 Milestone payments 2026 - 8,661 7,558 $ 123,740 $ 103,655
Schedule of Total Acquisition Related Contingent Consideration Liability	The following table presents a roll-forward of the total acquisition-related contingent consideration liability (in thousands): Fair Value Balance as of December 31, 2019 $ 103,655 Increase in fair value of contingent consideration 20,085 Balance as of September 30, 2020 $ 123,740

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): September 30, 2020 December 31, 2019 Laboratory equipment $ 11,763 $ 9,044 Computer equipment 892 779 Office equipment 942 883 Leasehold improvements 5,730 5,635 Construction in progress 5,094 3,460 24,421 19,801 Less: Accumulated depreciation and amortization (9,377 ) (7,262 ) $ 15,044 $ 12,539

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2020
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): September 30, December 31, Accrued employee compensation and benefits $ 4,918 $ 3,547 Accrued external research and development expenses 2,058 1,763 Accrued consultant and professional fees 1,358 1,390 Other 3,432 372 $ 11,766 $ 7,072

Incentive Stock Options and R_2

Incentive Stock Options and Restricted Stock (Tables)	9 Months Ended
	Sep. 30, 2020
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2019 (in thousands, except share and per share amounts): Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Intrinsic Value (in years) Outstanding as of December 31, 2019 8,646,378 $ 8.06 8.42 $ 3,687 Granted 2,992,734 $ 9.22 Exercised (800,076 ) $ 7.36 Forfeited (529,492 ) $ 8.59 Outstanding as of September 30, 2020 10,309,544 $ 8.42 8.04 $ 55,026 Exercisable as of September 30, 2020 4,915,617 $ 7.96 7.23 $ 27,795 Vested and expected to vest as of September 30, 2020 10,309,544 $ 8.42 8.04 $ 55,026
Summary of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-Date Fair Value of Stock Option Granted	The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees and directors: Nine Months Ended 2020 2019 Risk-free interest rate 0.77 % 2.39 % Expected term (in years) 6.1 6.0 Expected volatility 68.7 % 73.1 % Expected dividend yield 0 % 0 %
Summary of Restricted Stock Activity	The following table summarizes the Company’s restricted stock activity since December 31, 2019: Number of Shares Weighted Average Grant-Date Fair Value Unvested restricted common stock outstanding as of December 31, 2019 34,168 $ 1.28 Forfeited restricted common stock — $ — Vested restricted common stock (34,168 ) $ 1.28 Unvested restricted common stock outstanding as of September 30, 2020 — $ —
Summary of Stock-Based Compensation Expense	Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows (in thousands): Three Months Ended Nine Months Ended 2020 2019 2020 2019 Research and development expenses $ 1,578 $ 1,301 $ 7,123 $ 3,454 General and administrative expenses 2,077 1,598 5,718 4,954 $ 3,655 $ 2,899 $ 12,841 $ 8,408

Net Income (Loss) per Share (Ta

Net Income (Loss) per Share (Tables)	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Summary of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	Basic and diluted net income (loss) per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended Nine Months Ended 2020 2019 2020 2019 Basic net income (loss) per common share: Numerator: Net income (loss) $ 17,344 $ (21,227 ) $ (33,226 ) $ (82,257 ) Denominator: Weighted average common shares 73,183,923 51,891,157 65,187,435 48,574,275 Net income (loss) per share—basic $ 0.24 $ (0.41 ) $ (0.51 ) $ (1.69 ) Diluted net income (loss) per common share: Numerator: Net income (loss) $ 17,344 $ (21,227 ) $ (33,226 ) $ (82,257 ) Denominator: Weighted average common shares 76,440,293 51,891,157 65,187,435 48,574,275 Net income (loss) per share—diluted $ 0.23 $ (0.41 ) $ (0.51 ) $ (1.69 )
Schedule of Potential Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended Nine Months Ended 2020 2019 2020 2019 Options to purchase common stock — 8,568,506 10,309,544 8,568,506 Unvested restricted common stock — 72,677 — 72,677 — 8,641,183 10,309,544 8,641,183

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Schedule of Lease Cost	The components of lease cost were as follows (dollar amounts in thousands): Nine Months Ended 2020 2019 Lease cost Operating lease cost $ 3,397 $ 2,019 Total lease cost $ 3,397 $ 2,019 Other information Operating cash flows from operating leases $ 3,022 $ 1,933 Operating lease liabilities arising from obtaining right-of-use $ 53,778 — Weighted-average remaining lease term 5 years 8 years Weighted-average discount rate 11.9 % 17.5 %
Schedule of maturities of operating lease liabilities	As of September 30, 2020, maturities of operating lease liabilities are as follows (in thousands): September 30, 2020 $ 7,784 2021 14,737 2022 15,178 2023 15,591 2024 16,050 2025 and thereafter 19,164 Total future minimum lease payments 88,504 Less: imputed interest (23,074 ) Present value of lease liabilities $ 65,430
Schedule of Future Minimum Rental Commitments for Operating Leases	As of December 31, 2019, minimum rental commitments under the real estate lease was as follows (in thousands): December 31, 2020 $ 2,659 2021 2,737 2022 2,818 2023 2,860 2024 2,937 2025 and thereafter 10,160 Total future minimum lease payments 24,171 Less: imputed interest (11,557 ) Present value of lease liabilities $ 12,614

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	Jul. 20, 2020USD ($)$ / sharesshares	Sep. 30, 2020USD ($)Diseaseshares	Jun. 30, 2020USD ($)$ / sharesshares	Mar. 31, 2020USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Jun. 30, 2020USD ($)$ / sharesshares	Sep. 30, 2020USD ($)SubsidiaryDiseaseshares	Sep. 30, 2019USD ($)	Mar. 13, 2020USD ($)	Dec. 31, 2019USD ($)Diseaseshares
Initial Public Offering [Line Items]
Number of wholly owned subsidiaries | Subsidiary									2
Accumulated deficit		$ 392,722							$ 392,722			$ 359,496
Net loss		17,344	$ (36,288)	$ (14,282)	$ (21,227)	$ (27,832)	$ (33,198)		(33,226)	$ (82,257)
Stock available to be sold under sales agreement		$ 62,100							$ 62,100
Common stock issued | shares		74,251,559							74,251,559			60,022,067
Cash, cash equivalents and short-term investments		$ 674,100							$ 674,100
Proceeds from issuance of common stock									$ 154,292	$ 84,600
Sale from time to time of equity and debt securities											$ 350,000
Sanofi Pasteur Inc [Member]
Initial Public Offering [Line Items]
Maximum number of infectious disease pathogens for vaccine development		10							10
Second Sanofi Amendment Agreement [Member]
Initial Public Offering [Line Items]
Additional upfront payment	$ 300,000
Securities Purchase Agreement [Member]
Initial Public Offering [Line Items]
Shares Issued, Price Per Share | $ / shares	$ 25.59
Shares issued premium percent	50.00%
Stock Issued During Period, Value, New Issues	$ 125,000
Shares issued through public offering | shares	4,884,434
Sanofi Pasteur Collaboration and Licensing Agreement [Member] | Maximum [Member]
Initial Public Offering [Line Items]
Number of infectious disease pathogens for vaccine development | Disease		7							7
Sanofi Pasteur Collaboration and Licensing Agreement [Member] | Minimum [Member]
Initial Public Offering [Line Items]
Number of infectious disease pathogens for vaccine development | Disease		3							3
Sanofi Pasteur Collaboration and Licensing Agreement [Member] | Sanofi Pasteur Inc [Member]
Initial Public Offering [Line Items]
Number of infectious disease pathogens | Disease												6
Open Market Sale Agreement [Member]
Initial Public Offering [Line Items]
Payments of other offering expenses									$ 200
Payment of issuance commissions									$ 1,100
Issuance of common, shares | shares		0							2,863,163
Proceeds from issuance of common stock									$ 37,900
Market sale aggreement of common stock		$ 100,000							$ 100,000
Public Offering [Member]
Initial Public Offering [Line Items]
Payments of other offering expenses								$ 500
Common stock issued | shares			5,681,819					5,681,819
Gross proceeds from issuance of public offering								$ 125,000
Underwriting discounts and commissions								$ 7,500
Shares issued through public offering | shares								6,824,992
Common stock issued and sold, per share | $ / shares			$ 22					$ 22

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) $ / shares in Units, $ in Millions	Jul. 20, 2020USD ($)$ / sharesshares	Sep. 30, 2020USD ($)Disease	Sep. 30, 2020USD ($)Disease	Sep. 30, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized as a cumulative catch-up adjustment			$ 30.9
Second Sanofi Amendment Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional Upfront Payment	$ 300
Securities Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued through public offering | shares	4,884,434
Shares Issued, Price Per Share | $ / shares	$ 25.59
Shares issued premium percent	50.00%
Shares issued during the period value	$ 125
Number of trading days considered for ascertaining premium amount	20 days
Original Sanofi Agreement And Second Sanofi Amendment Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Increase in transaction price		$ 491.8	534.7
Non-refundable upfront payment			$ 345
Sanofi Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of infectious disease pathogens for vaccine development | Disease		5	5
Upfront payment received		$ 45	$ 45
Technology and process transfer milestone payment receivable		10	10
Estimated reimbursable employee cost		71.8	71.8
Estimated reimbursable development cost		155.7	155.7
Estimated milestone payments		62	62
Remaining performance obligation, expected to be recognized as revenue		$ 589.2	589.2
Revenue recognized from contract liabilities			$ 3.5	$ 1.8
Sanofi Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of infectious disease pathogens for vaccine development | Disease		6	6
Sanofi Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum number of infectious disease pathogens for vaccine development		10	10
Sanofi Collaboration and License Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of infectious disease pathogens for vaccine development | Disease		7	7
Sanofi Collaboration and License Agreement [Member] | Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of infectious disease pathogens for vaccine development | Disease		3	3
Sublicense Second Amendment Agreement With Sanofi [Member] | Massachusetts Institute Of Technology Exclusive Patent License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional upfront payment receivable		$ 75	$ 75
Milestone payment receivable upon the achievement of additional specified regulatory development manufacturing and commercial milestones		1,900	1,900
Lumpsum or one time manufacturing milestone amount receivable		200	200
Maximum development and regulatory milestone payment receivable		148	148
Sars Cov2 Licensed Filed [Member] | Sublicense Second Amendment Agreement With Sanofi [Member] | Massachusetts Institute Of Technology Exclusive Patent License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum development and regulatory milestone payment receivable		$ 250	250
Additional Paid-in Capital [Member] | Securities Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued during the period value			$ 51.2

Collaboration Agreement - Summa

Collaboration Agreement - Summary of Collaboration Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration revenue	$ 66,446	$ 1,266	$ 87,420	$ 3,914

Collaboration Agreement - Balan

Collaboration Agreement - Balance of Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Contract liabilities
Deferred revenue	$ 335,211	$ 43,356

Intangible Assets and Goodwil_2

Intangible Assets and Goodwill - Summary of Definite-Lived Intangible Assets Subject to Amortization and Indefinite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, accumulated amortization		$ (7,003)		$ (2,747)
Definite-lived intangible assets, Impairment Charge	$ (18,559)	0	$ (18,559)	(18,559)
Total intangible assets, gross carrying amount		88,283		106,842
Total intangible assets, net carrying amount		$ 81,280		$ 85,536
In-Process Research and Development [Member] | MRT Product [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, estimated life		6 years		8 years
Definite-lived intangible assets, gross carrying amount		$ 45,992		$ 45,992
Definite-lived intangible assets, accumulated amortization		(7,003)		(2,747)
Definite-lived intangible assets, Impairment Charge		0		0
Definite-lived intangible assets, net carrying amount		38,989		43,245
In-Process Research and Development [Member] | Cystic Fibrosis [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, accumulated amortization		0
Definite-lived intangible assets, Impairment Charge		0
Indefinite-lived intangible assets, gross carrying amount/net carrying amount		$ 42,291		42,291
In-Process Research and Development [Member] | OTC Deficiency [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, Impairment Charge	$ (18,600)			(18,559,000)
Indefinite-lived intangible assets, gross carrying amount/net carrying amount				$ 18,559

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Finite Lived Intangible Assets [Line Items]
indefinite lived intangible assets Impairment charge		$ 18,559,000	$ 0	$ 18,559,000	$ 18,559,000
Goodwill, Impairment Charges			0	0
Goodwill	$ 21,359,000		21,359,000		21,359,000
Goodwill changes			0
Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Goodwill	21,400,000		21,400,000
Shire's MRT Program [Member] | Sanofi Agreement [Member]
Finite Lived Intangible Assets [Line Items]
Estimated amortization expense of intangible assets for 2020	7,500,000		7,500,000
Estimated amortization expense of intangible assets for 2021	10,600,000		10,600,000
Estimated amortization expense of intangible assets for 2022	11,300,000		11,300,000
Estimated amortization expense of intangible assets for 2023	11,200,000		11,200,000
Estimated amortization expense of intangible assets for 2024	2,700,000		2,700,000
OTC Deficiency Program [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 0	400,000	$ 4,300,000	$ 1,200,000
In Process Research and Development [Member] | OTC Deficiency [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
indefinite lived intangible assets Impairment charge		$ 18,600,000			$ 18,559,000,000

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Total, assets	$ 636,615	$ 160,689
Liabilities:
Total, liabilities	123,740	103,655
Fair Value, Inputs, Level 1
Assets:
Total, assets	0
Liabilities:
Total, liabilities	0
Fair Value, Inputs, Level 2
Assets:
Total, assets	636,615	160,689
Liabilities:
Total, liabilities	0
Fair Value, Inputs, Level 3
Assets:
Total, assets	0
Liabilities:
Total, liabilities	123,740	103,655
Money Market Funds
Assets:
Total, assets	636,615	56,591
Money Market Funds | Fair Value, Inputs, Level 1
Assets:
Total, assets	0
Money Market Funds | Fair Value, Inputs, Level 2
Assets:
Total, assets	636,615	56,591
Money Market Funds | Fair Value, Inputs, Level 3
Assets:
Total, assets	0
U.S. Government Agency Bonds
Assets:
Total, assets	0	104,098
U.S. Government Agency Bonds | Fair Value, Inputs, Level 1
Assets:
Total, assets	0
U.S. Government Agency Bonds | Fair Value, Inputs, Level 2
Assets:
Total, assets	0	104,098
U.S. Government Agency Bonds | Fair Value, Inputs, Level 3
Assets:
Total, assets	0
Contingent Consideration
Liabilities:
Total, liabilities	123,740	103,655
Contingent Consideration | Fair Value, Inputs, Level 1
Liabilities:
Total, liabilities	0
Contingent Consideration | Fair Value, Inputs, Level 2
Liabilities:
Total, liabilities	0
Contingent Consideration | Fair Value, Inputs, Level 3
Liabilities:
Total, liabilities	$ 123,740	$ 103,655

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Schedule of Unobservable Inputs and Fair Value Components of Contingent Consideration (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Earnout Payments | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2026	2026
Earnout Payments | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2039	2039
Milestone Payments | Minimum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2026	2026
Milestone Payments | Maximum [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Projected Year of Payment	2032	2032
Contingent Consideration
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 123,740	$ 103,655
Contingent Consideration | Earnout Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	115,079	96,097
Contingent Consideration | Milestone Payments
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value at	$ 8,661	$ 7,558

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Schedule of Total Acquisition Related Contingent Consideration Liability (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition, Contingent Consideration [Line Items]
Beginning Balance			$ 103,655
Discontinuation of MRT5201 and Increase in fair value of contingent consideration	$ 14,190	$ (19,834)	20,085	$ (3,243)
Ending Balance	$ 123,740		123,740
Contingent Consideration
Business Acquisition, Contingent Consideration [Line Items]
Discontinuation of MRT5201 and Increase in fair value of contingent consideration			$ 20,085

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 674,050,000	$ 84,580,000	$ 98,930,000
Contingent Consideration
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount Rate	12.5	13.5
Money Market Funds | Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 636,600,000	$ 56,600,000
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value, assets transfers into (out of) Level 3		0
Fair value,assets transfers from Level 2 to Level 1	0	0
Fair value,assets transfers from Level 1 to Level 2	0	0
Fair value, liabilities transfers from Level 1 to Level 2	0	0
Fair value, liabilities transfers from Level 2 to Level 1	0	0
Fair Value, liabilities transfers into (out of) Level 3	$ 0	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 24,421	$ 19,801
Less: Accumulated depreciation and amortization	(9,377)	(7,262)
Property and equipment, net	15,044	12,539
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	11,763	9,044
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	892	779
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	942	883
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,730	5,635
Construction In Progress [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 5,094	$ 3,460

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 0.8	$ 0.6	$ 2.1	$ 1.8

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 4,918	$ 3,547
Accrued external research and development expenses	2,058	1,763
Accrued consultant and professional fees	1,358	1,390
Other	3,432	372
Total accrued expenses	$ 11,766	$ 7,072

Incentive Stock Options and R_3

Incentive Stock Options and Restricted Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2020	Jun. 15, 2018	Mar. 07, 2018	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 01, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 847		$ 847
Unrecognized compensation cost related to unvested stock-based awards					$ 27,200
Unrecognized compensation cost, period for recognition					2 years 6 months
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vested Stock options for purchase of common stock					10,309,544
Intrinsic value of stock options, exercised					$ 10,700	$ 600
Weighted average grant-date fair value					$ 5.65	$ 5.58
2018 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Sharebased Compensation Arrangement by Sharebased Payment Award Shares Cancelled in Period					7,330,005
Number of common shares reserved for issuance							4,829,847
Number of shares remaining available for issuance			360,514
Increase in common stock issued	2,400,829
Percentage threshold of outstanding shares under the plan		4.00%
Percentage of exercise price per share of fair market value		100.00%
Expiration period of stock options after grant date		10 years
2018 Stock Incentive Plan [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock issued during period under the plan		3,349,582
Stock options for purchase of common stock held, exercisable period		1 year
2018 Stock Incentive Plan [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options for purchase of common stock held, exercisable period		4 years
2018 Stock Incentive Plan [Member] | Share-based Compensation Award, Tranche One [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance			2,512,187
2018 Stock Incentive Plan [Member] | Share-based Compensation Award, Tranche Two [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance			1,013,167
2018 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares reserved for issuance			418,697					870,096
Percentage threshold of outstanding shares under the plan			1.00%
2018 Employee Stock Purchase Plan [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock issued during period under the plan			837,395
2016 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Sharebased Compensation Arrangement by Sharebased Payment Award Shares Cancelled in Period					99,329
Percentage of exercise price per share of fair market value		100.00%
Expiration period of stock options after grant date		10 years
Stock options for purchase of common stock held, exercisable period		4 years
Research and Development Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense					$ 2,400
EVP and Founder [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options for purchase of common stock held					550,278
Vested Stock options for purchase of common stock					176,266
Stock options for purchase of common stock held, exercisable period					18 months

Incentive Stock Options and R_4

Incentive Stock Options and Restricted Stock - Summary of Stock Option Activity (Detail) - Stock Options [Member] $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares
Number of shares
Number of shares, beginning balance | shares	8,646,378
Number of shares, granted | shares	2,992,734
Number of shares, exercised | shares	(800,076)
Number of shares, forfeited | shares	(529,492)
Number of shares, ending balance | shares	10,309,544	8,646,378
Number of shares, exercisable | shares	4,915,617
Number of shares, vested and expected to vest, ending balance | shares	10,309,544
Weighted average exercise price
Weighted average exercise price, beginning balance | $ / shares	$ 8.06
Weighted average exercise price, granted | $ / shares	9.22
Weighted average exercise price, exercised | $ / shares	7.36
Weighted average exercise price, forfeited | $ / shares	8.59
Weighted average exercise price, ending balance | $ / shares	8.42	$ 8.06
Weighted average exercise price, exercisable | $ / shares	7.96
Weighted average exercise price, vested and expected to vest, ending balance | $ / shares	$ 8.42
Weighted average remaining contractual term
Weighted average remaining contractual term, ending balance	8 years 14 days	8 years 5 months 1 day
Weighted average remaining contractual term, exercisable	7 years 2 months 23 days
Weighted average remaining contractual term, vested and expected to vest, ending balance	8 years 14 days
Intrinsic value
Intrinsic value, ending balance | $	$ 55,026	$ 3,687
Intrinsic value, exercisable | $	27,795
Intrinsic value, vested and expected to vest, ending balance | $	$ 55,026

Incentive Stock Option and Rest

Incentive Stock Option and Restricted Stock - Summary of Assumptions Used in Black-Scholes Option-Pricing Model to Determine Grant-Date Fair Value of Stock Option Granted (Detail) - Employees And Directors [Member] - Stock Options [Member]	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	0.77%	2.39%
Expected term (in years)	6 years 1 month 6 days	6 years
Expected volatility	68.70%	73.10%
Expected dividend yield	0.00%	0.00%

Incentive Stock Option and Re_2

Incentive Stock Option and Restricted Stock - Summary of Restricted Stock Activity (Detail) - Restricted Common Stock [Member]	9 Months Ended
	Sep. 30, 2020$ / sharesshares
Number of shares
Number of shares, beginning balance | shares	34,168
Number of shares, forfeited | shares	0
Number of shares, vested | shares	(34,168)
Number of shares, ending balance | shares	0
Weighted average grant-date fair value
Weighted average grant-date fair value, beginning balance | $ / shares	$ 1.28
Weighted average grant-date fair value, forfeited | $ / shares	0
Weighted average grant-date fair value, vested | $ / shares	1.28
Weighted average grant-date fair value, ending balance | $ / shares	$ 0

Incentive Stock Option and Re_3

Incentive Stock Option and Restricted Stock - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock-based compensation expense	$ 3,655	$ 2,899	$ 12,841	$ 8,408
Research and Development Expenses [Member]
Stock-based compensation expense	1,578	1,301	7,123	3,454
General and Administrative Expenses [Member]
Stock-based compensation expense	$ 2,077	$ 1,598	$ 5,718	$ 4,954

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating Loss Carryforwards [Line Items]
Income tax benefits		$ (486)
Shires Mrt Program [Member]
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ 0	$ 500

Net Income (Loss) per Share - S

Net Income (Loss) per Share - Summary of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net loss	$ 17,344	$ (36,288)	$ (14,282)	$ (21,227)	$ (27,832)	$ (33,198)	$ (33,226)	$ (82,257)
Denominator:
Weighted average common shares outstanding—basic	73,183,923			51,891,157			65,187,435	48,574,275
Net income (loss) per share—basic	$ 0.24			$ (0.41)			$ (0.51)	$ (1.69)
Weighted average common shares outstanding— diluted	76,440,293			51,891,157			65,187,435	48,574,275
Net income (loss) per share—diluted	$ 0.23			$ (0.41)			$ (0.51)	$ (1.69)

Net Income (Loss) per Share - A

Net Income (Loss) per Share - Additional Information (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	0	8,641,183	10,309,544	8,641,183
Restricted Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	0	72,677	0	72,677
Restricted Common Stock [Member] | Weighted Average [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share		99,468		147,842

Net Income (Loss) per Share -_2

Net Income (Loss) per Share - Summary of Potential Common Shares Excluded from Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	0	8,641,183	10,309,544	8,641,183
Stock Options [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	0	8,568,506	10,309,544	8,568,506
Restricted Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of basic net loss per share	0	72,677	0	72,677

Leases - Additional Information

Leases - Additional Information (Details) $ in Thousands	Mar. 31, 2018	Jul. 31, 2017USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019	Jun. 30, 2017USD ($)ft²
Lessee Lease Description [Line Items]
Lease expiration period	Mar. 31, 2023		Mar. 31, 2023
Remaining term of lease			5 years		8 years
Operating lease right of use assets			$ 75,650	$ 10,400
Operating lease, liability			$ 65,430	$ 12,614
Lexington, Massachusetts [Member]
Lessee Lease Description [Line Items]
Office and laboratory space | ft²						59,000
Monthly lease payments		$ 200
Percentage of annual increase in operating lease		3.00%
Cash deposit collateral						$ 1,000
Remaining term of lease			9 years
Operating Lease Term Of Contract	5 years					10 years
Suite Retention And Development Agreement [Member]
Lessee Lease Description [Line Items]
Commitment to Build Out Cost			$ 6,000
Shared Overage cost commitment			11,000
Albany Molecular Research, Inc. ("AMRI") [Member] | Suite Retention And Development Agreement [Member]
Lessee Lease Description [Line Items]
Monthly lease payments			1,000
Future Build Out Costs			4,100
Operating lease right of use assets			66,600
Operating lease, liability			$ 53,800
Operating Lease Term Of Contract			5 years
Operating Lease Renewal Term			3 years
Operating lease option to extend description			the Company has the right to extend for an additional three years.
Percentage Of Increase In The Monthly Rental Expense			3.00%
Albany Molecular Research, Inc. ("AMRI") [Member] | Suite Retention And Development Agreement [Member] | Other Noncurrent Assets [Member]
Lessee Lease Description [Line Items]
Payments For Build Out Costs			$ 12,800

Leases - Schedule of Lease Cost

Leases - Schedule of Lease Cost (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Lease Cost
Operating lease cost	$ 3,397	$ 2,019
Total lease cost	3,397	2,019
Operating cash flows from operating leases	3,022	$ 1,933
Operating lease liabilities arising from obtaining right-of-use assets	$ 53,778
Weighted-average remaining lease term	5 years	8 years
Weighted-average discount rate	11.90%	17.50%

Leases - Schedule of Maturities

Leases - Schedule of Maturities of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Leases [Abstract]
2020	$ 7,784
2021	14,737
2022	15,178
2023	15,591
2024	16,050
2025 and thereafter	19,164
Total future minimum lease payments	88,504
Less: imputed interest	(23,074)	$ (11,557)
Present value of lease liabilities	$ 65,430	$ 12,614

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Rental Commitments for Operating Leases (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Leases [Abstract]
2020		$ 2,659
2021		2,737
2022		2,818
2023		2,860
2024		2,937
2025 and thereafter		10,160
Total future minimum lease payments		24,171
Less: imputed interest	$ (23,074)	(11,557)
Present value of lease liabilities	$ 65,430	$ 12,614

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Loss Contingencies [Line Items]
Research agreement, payable amount		$ 2,058		$ 2,058		$ 1,763
Research and development		26,344	$ 17,295	$ 76,785	$ 51,343
Research agreement, expiration period				2022-12
Research agreement, payment				$ 1,400
Roche Diagnostics Corporation Master Supply Agreement [Member]
Loss Contingencies [Line Items]
Raw material to purchase as percentage of demand				80.00%
Agreement extended date				Dec. 31, 2024
Commitment amount				$ 14,000
Purchase commitments, year 2021		3,500		3,500
Purchase commitments, year 2022		3,500		3,500
Purchase commitments, year 2023		3,500		3,500
Purchase commitments, year 2024		3,500		3,500
Research and development		100	1,800	2,700	5,400
MIT Research Agreement [Member]
Loss Contingencies [Line Items]
Research agreement, payable amount		0		0		$ 0
Research and development		300		1,000
Research agreement, committed amount	$ 4,100
Research agreement, expiration period	2022-12
MIT Exclusive Patent License Agreement [Member]
Loss Contingencies [Line Items]
Research and development		100	$ 100	100	100
Annual license maintenance payments		200		200
Payments of annual license maintenance fees				200	$ 200
MIT Exclusive Patent License Agreement [Member] | Sanofi Pasteur Inc [Member]
Loss Contingencies [Line Items]
Upfront payment received		700		700
MIT Exclusive Patent License Agreement [Member] | Milestone Payment One [Member]
Loss Contingencies [Line Items]
License agreement, milestone payments				1,375
MIT Exclusive Patent License Agreement [Member] | Milestone Payment Two [Member]
Loss Contingencies [Line Items]
License agreement, milestone payments				1,250
Sublicense Second Amendment Agreement With Sanofi [Member] | MIT Exclusive Patent License Agreement [Member] | Sanofi Pasteur Inc [Member]
Loss Contingencies [Line Items]
Additional upfront payment payable		$ 300,000		$ 300,000
Percentage of the premium payment to be received on share issue payable as additional upfront payment		50.00%		50.00%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Jun. 30, 2020	Sep. 30, 2019	May 31, 2019
Private Placement [Member] | Baupost Group LLC [Member]
Related Party Transaction [Line Items]
Sale of Stock, Number of Shares Issued in Transaction			2,352,941
Sale of Stock, Price Per Share			$ 8.50
Sale of Stock, Consideration Received on Transaction			$ 20
Public Offering [Member]
Related Party Transaction [Line Items]
Sale of Stock, Price Per Share	$ 22
Public Offering [Member] | Baupost Group LLC [Member]
Related Party Transaction [Line Items]
Sale of Stock, Number of Shares Issued in Transaction	500,000	5,000,000
Sale of Stock, Price Per Share	$ 22	$ 10
Sale of Stock, Consideration Received on Transaction	$ 11	$ 50

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - Leaseholds and Leasehold Improvements [Member] - Subsequent Event [Member] - Waltam Massauchets [Member] $ in Millions	Nov. 03, 2020USD ($)ft²
Subsequent Event [Line Items]
Area of the leased property | ft²	138,444
Operating lease term	10 years
Operating lease term of extension	10 years
Initial base rent	$ 5.7
Subsequent base rent	$ 8
Percent of annual increase in base rent	3.00%