The nonbinding decision from the agency's Vaccines and Related Biological Products Advisory Committee, an outside group of experts in infectious diseases and vaccines, is the last step before the FDA is likely to give the final OK to distribute the potentially lifesaving doses throughout the United States.
While the FDA does not have to follow the advisory committee's recommendation, the agency often takes its advice. Emergency use approval would mark a pivotal moment in the Covid-19 pandemic, which has infected more than 15.3 million Americans and killed at least 288,000 as of Wednesday, according to data compiled by Johns Hopkins University. Hospitals across the U.S. already have a higher load of Covid patients than ever before, and the country's outbreak is primed to set even more grim records.
Pfizer's vaccine would be the first approved for use in the U.S.
Here's what to expect:
The FDA advisory meeting is scheduled to run from 9 a.m. ET to 6 p.m. ET. The vote is likely to happen toward the end of the meeting. Before the vote, outside medical experts will assess Pfizer's clinical trial data and offer their opinions on the vaccine, including whether the benefits outweigh the risks for an emergency use authorization.
Such an authorization means the FDA will allow some people to receive the vaccine as the agency continues to evaluate data. It isn't the same as full approval, which can typically take months. So far, Pfizer has only submitted two months of follow-up safety data, but the agency usually requires six months for full approval.
The advisory committee, which is expected to include 23 members for the meeting, has already been reviewing documents sent by the FDA on Pfizer's vaccine, said Dr. Paul Offit, a voting member of the committee. Those documents were made available to the public on Tuesday. "The public will see everything that we see," Offit added.
The FDA will make a decision on whether to approve the vaccine for emergency use, which could come as early as Friday, James Hildreth, a member of the committee, told NBC's "Weekend Today" on Saturday. Health and Human Services Secretary Alex Azar told ABC News' "This Week" on Sunday that an authorization could come "within days" after the meeting.
The committee's recommendation is nonbinding, meaning the agency doesn't have to accept it. But it often does. FDA Commissioner Stephen Hahn said in September that he had "no intention" of overruling career scientists at the agency, including Peter Marks, who runs the division that oversees vaccine approvals. "I have complete and absolute confidence in the scientists at the FDA and the decision-making that they have here," he said Sept. 10.
The federal government is expected to ship 2.9 million doses of Pfizer's vaccine to jurisdictions across the nation within 24 hours after an emergency use authorization from the FDA, Army Gen. Gustave Perna, chief operations officer for Operation Warp Speed, said at a briefing Wednesday.
An additional 2.9 million doses will be held to be distributed 21 days later for second doses, he added.
States had already submitted early plans to the Centers for Disease Control and Prevention on how they intend to inoculate some 331 million Americans against Covid-19 once a vaccine is approved. The CDC has allocated $200 million to jurisdictions for vaccine preparedness, though much of that funding hasn't trickled down to the local level.
Initial doses of the vaccine will be limited as manufacturing ramps up, with officials predicting it will take months to immunize everyone in the U.S. who wants to be vaccinated. The vaccine is expected to be distributed in phases, with the CDC asking states to prioritize health-care workers and nursing homes first.
Dr. Moncef Slaoui, who is leading President Donald Trump's vaccine program Operation Warp Speed, has said the U.S. should be able to distribute enough vaccine doses to immunize 100 million Americans by the end of February, nearly a third of the U.S. population. He has said the entire U.S. population could be vaccinated against Covid-19 by June.
The FDA said Tuesday that patients who have had Covid-19 may still be at risk of reinfection and could benefit from the vaccine.
The World Health Organization said last week that researchers are still trying to determine how long an antibody response lasts after someone is infected with the virus. In a recent Oxford study, researchers found that people who have contracted the coronavirus are "highly unlikely" to contract the disease again for at least six months.
"We have seen the number of people infected continue to grow, but we're also seeing data emerge that protection may not be lifelong, and therefore we may see reinfections begin to occur," Dr. Mike Ryan, executive director of the WHO's health emergencies program, said Dec. 4. "So the question is: What are the levels of protection in society?"
Most people don't have serious side effects from vaccines, and severe reactions are extremely rare, according to the Department of Health and Human Services. Slaoui has said says Pfizer's and Moderna's vaccines are safe, with only 10% to 15% of volunteers reporting side effects that were "significantly noticeable."
Participants in Moderna's and Pfizer's coronavirus vaccine trials told CNBC in September that they were experiencing high fever, body aches, bad headaches, daylong exhaustion and other symptoms after receiving the shots. While the symptoms were uncomfortable, and at times intense, the participants said they often went away after a day, sometimes sooner, and that it was better than getting Covid-19.
The FDA notes there is currently insufficient data to make conclusions about the safety of the vaccine in children under age 16, pregnant women and people with compromised immune systems.
The U.K.'s drug regulator warned Wednesday that people who have a history of "significant" allergic reactions should not receive the coronavirus vaccine developed by Pfizer.