| Document and Entity Information | Feb. 10, 2022 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001786255 |
| Document Type | 8-K |
| Document Period End Date | Feb. 10, 2022 |
| Entity Registrant Name | ICOSAVAX, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-40655 |
| Entity Tax Identification Number | 82-3640549 |
| Entity Address, Address Line One | 1616 Eastlake Avenue E. |
| Entity Address, Address Line Two | Suite 208 |
| Entity Address, City or Town | Seattle |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98102 |
| City Area Code | (206) |
| Local Phone Number | 737-0085 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.0001 per share |
| Trading Symbol | ICVX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
