The COVID-19 vaccine candidate developed by BioNTech /zigman2/quotes/214419716/composite BNTX +0.0096% and Pfizer Inc. /zigman2/quotes/202877789/composite PFE -0.37% likely has a "favorable safety profile," according to documents published Tuesday by the Food and Drug Administration. In premarket trading on Tuesday, BioNTech's stock was up 1.3%, and shares of Pfizer gained 0.9%. As part of the FDA approval process, a committee of independent medical experts will meet Thursday to discuss and then vote on whether the regulator should approve or authorize what may be the first COVID-19 vaccine to be made available in the U.S. The FDA document was shared publicly in advance of that meeting. The companies so far have released limited data about how the vaccine performed in clinical trials beyond the 95% efficacy rate. The FDA said the number of COVID-19 infections among trial participants who received the placebo or the vaccine was the same until 14 days after the first dose was administered. At that point, participants who were vaccinated were less likely to contract COVID-19. Though people who received the first dose of the two-dose vaccine had an efficacy rate of 88.9% after the first dose, the FDA said it does not support utilization of a single dose. The agency also cited several unknowns about the vaccine, including durability of protection, how the vaccine protects children and people who are immunocompromised, such as those with HIV/AIDS, and if the vaccine can prevent reinfection in people who have already been infected with the coronavirus. The advisory committee meeting is scheduled to be webcast Thursday starting at 9 a.m. ET. The S&P 500 /zigman2/quotes/210599714/realtime SPX +0.63% is up 14.2% so far this year.