The meeting of outside advisers to the Food and Drug Administration represented the next-to-last hurdle before the expected start of the biggest vaccination campaign in U.S. history. Depending on how fast the FDA signs off on the panel’s recommendation, shots could begin within days.

The FDA panel functions like a science court. During the scheduled daylong session, it was expected to debate and pick apart the data — in public and live-streamed — on whether the vaccine is safe and effective enough to be cleared for emergency use.

“The American public demands and deserves a rigorous, comprehensive and independent review of the data,” said FDA’s Dr. Doran Fink, who described agency scientists working nights, weekends and over Thanksgiving to get that done.

With unprecedented interest in the normally obscure panel, the FDA broadcast the meeting via Youtube, and thousands logged on to follow the discussion.

The FDA is not required to follow the committee’s advice but is widely expected to do so. Once that happens, the U.S. will begin shipping millions of doses of the shot.

The meeting came as the coronavirus continues surging across much of the world, claiming more than 1.5 million lives, including more than 289,000 in the U.S.

Hanging over the meeting is a warning from British officials that people with a history of serious allergic reactions shouldn’t get the vaccine. Government officials there are investigating two reports of reactions that occurred when the country began mass vaccinations on Tuesday.

Still, a positive recommendation and speedy U.S. approval appeared nearly certain after FDA scientists issued an overwhelmingly positive initial review of the vaccine earlier this week.

“The data presented in the briefing report were consistent with what we heard before and are really exciting,” said Dr. William Moss, head of Johns Hopkins University’s International Vaccine Access Center. “Nothing that I see would delay an emergency use authorization.”

The meeting also represented an opportunity for regulators to try to boost public confidence in the breakneck development process that has produced the Pfizer-BioNTech vaccine and a string of other upcoming shots in less than a year.

The FDA has also faced weeks of criticism from President Donald Trump for not rushing out a vaccine before Election Day.

“There have been a lot of questions about why it takes us so long or ‘are we being rigorous enough?’” said FDA Commissioner Stephen Hahn in an interview. “I’m hoping that people will see with our transparency that we have taken a very rigorous stance on this.”

Hahn said the agency had already teed up the process to authorize the vaccine by filling out all the legal paperwork in advance, regardless of the ultimate decision.

On Thursday’s agenda:

Read Also: UK probing if allergic reactions linked to Pfizer vaccine

RARE ADVERSE REACTIONS

The FDA uncovered no major safety problems in its review of Pfizer’s 44,000-person study, including no allergic reactions of the type reported in Britain. But such studies can’t detect rare problems that might only affect a tiny slice of the general population.

FDA reviewers noted four cases of Bell’s palsy that occurred among people getting the vaccine. They concluded the cases were probably unrelated to the vaccine because they occurred at rates that would be expected without any medical intervention. But the agency did say cases of the nerve disorder should be tracked, given that other vaccines can cause the problem.

“I think we have to be upfront, without scaring people, that we don’t know yet about any potential, rare, long-term adverse events,” Moss said.

Read Also: Canada health regulator approves Pfizer's COVID-19 vaccine

EFFICACY QUESTIONS

The FDA found the vaccine highly effective across various demographic groups. But it’s unclear how well the vaccine works in people with HIV and other immune-system disorders.

The study excluded pregnant women, but experts will tease apart the data for any hints in case women get vaccinated before realizing they’re pregnant.

A study of children as young as 12 is underway.

IMPACT OF EMERGENCY AUTHORIZATION

Answering some of these questions will require keeping Pfizer’s study going for many more months.

When the FDA panel met in October, experts warned against allowing trial participants who received placebos to switch and get the real vaccine immediately after it receives the FDA’s emergency OK. Doing that could make it impossible to get answers to long-term questions, including how long the protection lasts.

Pfizer and BioNTech say they want to allow such participants to get the vaccine either upon request or, at the latest, after six months of follow-up. The FDA still hasn’t made clear if it will accept that approach.

“FDA is adamant that they want these trials completed,” said Norman Baylor, former director of FDA’s vaccine office.