General and administrative expense. General and administrative expense was $11.7 million and $7.2 million for the nine months ended September 30, 2020 and 2019. The significant increase in general and administrative expense was primarily due to stock-based compensation expense. On July 28, 2020, and August 24, 2020, the Company granted, as fulfillment of a contractual obligation with respect to a milestone which has been met, options to purchase an aggregate of 3,412,100 shares of common stock to the Chief Executive Officer. These options were vested and exercisable upon grant and therefore contributed $4.3 million to the significant increase in general and administrative expense.
Change in fair value of contingent consideration. The change in fair value of contingent consideration was $1.0 million for the nine months ended September 30, 2020, compared to $0.7 million in the nine months ended September 30, 2019. The change in the 2020 period primarily reflects the re-calculation of discounted cash flows for the passage of time and milestone achievement.
Total non-operating income (loss). Total non-operating loss was ($0.7) million for the nine months ended September 30, 2020, compared to income of $0.3 million for the nine months ended September 30, 2019. The change of ($1.0) million primarily consists changes in fair value related to warrants of ($1.0) million.
Net loss attributable to Heat Biologics, Inc. We had a net loss attributable to Heat Biologics, Inc. of $19.6 million, or ($0.20) per basic and diluted share for the nine months ended September 30, 2020 compared to a net loss of $16.7 million, or ($0.50) per basic and diluted share for the nine months ended September 30, 2019.
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. During May 2018, we closed a public offering of shares of our common stock and warrants to purchase shares of our common stock in which we received net proceeds of approximately $18.8 million and after the closing of the offering, an additional $4.8 million from the exercise of 3,054,667 warrants issued in this offering. During November 2018, we closed a public offering of shares of our common stock and warrants to purchase shares of our common stock in which we received net proceeds of approximately $12.7 million. For the year ended December 31, 2018, we received net proceeds of approximately $3.8 million from sales of our common stock in at-the-market offerings. 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 nine months ended September 30, 2020, we received net proceeds of $113.4 million from the sale of 91,399,427 shares of our common stock in at-the-market offerings. As of September 30, 2020, we had an accumulated deficit of $124.2 million. We had net losses of $20.4 million and $16.6 million for the years ended December 31, 2019 and 2018, respectively. We had net losses of $19.8 million and $17.1 million for the nine months ended September 30, 2020 and 2019, respectively. Subsequent to the quarter ended September 30, 2020, we have issued an additional 830,667 shares of common stock in “at-the-market” offerings and received $1.0 million of net proceeds.
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 2023, we will need to obtain substantial additional future funding in connection with our future planned clinical trials. 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, there can be no assurance that we will receive such grant funding or if received, the amount of such grant funding. Adequate