Total non-operating income (loss). Other income was $0.1 million for the six months ended June 30, 2021, compared to a loss of $(0.9) million for the six months ended June 30, 2020. The change of $1.0 million primarily consists of a decrease in extinguishment expense and changes in fair value related to warrants of ($1.0) million, and an increase in interest income on cash and short-term investment balances of $0.3 million and $(0.3) million of unrealized losses on short-term investment balances.
Net loss attributable to Heat Biologics, Inc. We had a net loss attributable to Heat Biologics, Inc. of $14.1 million, or ($0.57) per basic and diluted share for the six months ended June 30, 2021 compared to a net loss of $10.7 million, or ($1.04) per basic and diluted share for the six months ended June 30, 2020.
LIQUIDITY AND CAPITAL RESOURCES
Sources of Liquidity
Since our inception in June 2008, we have incurred significant losses and we have financed our operations with net proceeds from the private placement of our preferred stock, common stock and debt. Since our initial public offering, we have primarily financed our operations with net proceeds from the public offering of our securities and to a lesser extent, the proceeds from the exercise of warrants. On January 21, 2020, we closed an underwritten public offering of shares of our common stock and warrants to purchase shares of our common stock pursuant to which we received net proceeds of approximately $6.4 million. For the year ended December 31, 2020 we received net proceeds of approximately $114.4 million from sales of our common stock in at-the-market offerings. For the six months ended June 30, 2021 we received net proceeds of $25.6 million from the sale of 2,106,027 shares of our common stock in at-the-market offerings. We had no sales of our common stock in at-the-market offerings for the three months ended June 30, 2021. As of June 30, 2021, we had an accumulated deficit of $144.7 million. We had net losses of $26.4 million and $20.4 million for the years ended December 31, 2020 and 2019, respectively. We had net losses of $14.2 million and $10.9 million for the six months ended June 30, 2021 and 2020, respectively.
We expect to incur significant expenses and continued losses from operations for the foreseeable future. We expect our expenses to increase in connection with our ongoing activities, particularly as we continue the research and development and advance our clinical trials of, and seek marketing approval for, our product candidates and as we add to our product candidate pipeline. In addition, if we obtain marketing approval for any of our product candidates, we expect to incur significant commercialization expenses related to product sales, marketing, manufacturing, and distribution. Although we currently have sufficient funds to complete our Phase 2 clinical trials, as currently planned, and expect that we will have sufficient funds to fund our operations into 2024, we will need to obtain substantial additional future funding in connection with our future planned clinical trials, the manufacturing facility that we intend to build in San Antonio, Texas and any new programs or ventures we pursue. While we are currently funding vaccine development and preclinical studies, we do not expect to use significant corporate resources to advance our COVID-19 program. We are applying for several large grants to support clinical development of this program and are engaged in collaboration discussions, which we believe may provide attractive and non-dilutive pathways to help accelerate development of our COVID-19 program; however, to date we have not received any grant funding for such program and there can be no assurance that we will receive such grant funding or if received, the amount of such grant funding. Adequate additional financing may not be available to us on acceptable terms, or at all. If we are unable to raise capital when needed or on attractive terms, we would be forced to delay, reduce or eliminate our research and development programs or any future commercialization efforts. To meet our capital needs, we are considering multiple alternatives, including, but not limited to, additional equity financings, which include sales of our common stock under at-the-market offerings, if available, debt financings, partnerships, collaborations and other funding transactions. This is based on our current estimates, and we could use our available capital resources sooner than we currently expect. We will need to generate significant revenues to achieve profitability, and we may never do so. As of June 30, 2021, we had approximately $122.5 million in cash and cash equivalents and short-term investments.
Cash Flows
Operating activities. The use of cash during the six months ended June 30, 2021 and 2020 resulted primarily from our net losses adjusted for non-cash charges and changes in components of working capital. Net cash used in operating activities during the six months ended June 30, 2021 was $11.0 million compared to $10.1 million during the same period in 2020.