Documents released by U.S. regulators Tuesday confirmed that Pfizer and BioNTech's COVID-19 vaccine was strongly protective against COVID-19 — offering the world's first detailed look at the evidence behind the shots.

The Food and Drug Administration posted its analysis online even as across the Atlantic, Britain on Tuesday began vaccinating its oldest citizens with the Pfizer-BioNTech shots. A 90-year-old grandmother, Margaret Keenan, received the first dose and said she felt "so privileged to be the first person vaccinated against COVID-19."

But the U.S. judges experimental vaccines in a unique way: On Thursday, the FDA will convene what's essentially a science court that will debate — in public and live-streamed — just how strong the data backing the shots really is.

A panel of independent scientists will pick apart the FDA's first-pass review before recommending whether the vaccine appears safe and effective enough for millions of Americans. The FDA, which typically follows the committee's advice, is expected to issue a decision in the days following the review. If given the green light, the first recipients would be health care workers and nursing home residents according to plans laid out by each state.

Pfizer CEO Albert Bourla declined to predict how quickly FDA might issue a decision.

"They need to take as much time as they need to feel comfortable. It's very important for the trust of the vaccine from the people," he said at a news conference in Geneva.

It's one of several vaccines in the pipeline being tested in the hope of ending the pandemic. The FDA later this month will consider a COVID-19 vaccine by Moderna and the National Institutes of Health.

Pfizer and its German partner BioNTech previously reported the shots appear 95% effective at preventing mild to severe COVID-19 disease in a large, ongoing study. That's based on the first 170 infections detected. Only eight of the infections were among volunteers given the real vaccine while the rest had received a dummy shot.

That was measured soon after study participants got their second dose. Still unknown is how long that protection lasts. "We're looking at the best possible data," Dr. Paul Offit of Children's Hospital of Philadelphia and an FDA adviser recently cautioned. "People worry, reasonably, how about six months later?"

The other critical issue: Safety. Pfizer has reported no serious side effects. Some recipients experience flu-like reactions — including fever, fatigue or muscle aches — especially after the required second dose. It's a sign the immune system is revving up, able to recognize and fight back if the real virus comes along.

Other questions on the FDA advisers' list:

1. How well do the shots protect people at high risk such as those over age 65 or those of any age with additional health problems such as obesity or heart disease?
2. Were the shots adequately tested in Black, Hispanic and other communities hard-hit by the pandemic, to know how well the vaccine works in those populations?
3. Does the vaccine protect against asymptomatic infection, or could people who were vaccinated still unknowingly spread the virus?
4. What should pregnant women be told about vaccination, since they weren't tested in Pfizer's study?

Studies in children as young as 12 are just beginning.

Emergency vaccinations could begin before Pfizer's 44,000-person study is complete, and answering some of those questions will require keeping that study going. Health authorities are wrestling with how to do so in a way that's fair to placebo recipients who justifiably would want to get the real vaccine.

That access "is top of mind for many," Pfizer and BioNTech recently wrote trial participants. The companies said they're exploring ways to let placebo recipients switch to the vaccine group once they meet eligibility criteria for early access — if they're health workers, for instance, or when the line opens for other essential workers or people over age 65.

On the safety front, study volunteers will be monitored for two years but even studies of tens of thousands of people can't spot a complication that only strikes 1 in a million. So the government also is gearing up for unprecedented monitoring of recipients once emergency vaccinations get underway.

The Pfizer-BioNTech vaccine is administered in two doses, with the second shot given 21 days after the first. 

The documents released Tuesday by the FDA suggested the possibility that the first injection might provide at least some protection, although it's unclear exactly how much or how long that would last. 

"Secondary efficacy analyses suggested benefit of the vaccine in preventing severe COVID-19, in preventing COVID-19 following the first dose, and in preventing COVID-19 in individuals with prior SARS-CoV-2 infection, although available data for these outcomes did not allow for firm conclusions," the report says.

"We don't know because that wasn't what was studied, but we're assuming that the titers — the antibody titers — won't become high enough to prevent coronavirus" after just the first shot, Dr. Dyan Hes, a pediatrician in New York City, told CBSN. "So the first one might be a primer for your immune system and the second one completes your immunization." 

Experts have raised concerns that the vaccination program's effectiveness could be undermined if people fail to return for their second dose. Moderna's vaccine candidate also requires two shots, 28 days apart. Another vaccine developed by Oxford and AstraZeneca, which has not yet applied for U.S. approval, requires two shots as well.

Dr. Hes also noted that people will need to continue wearing masks for some time, even after getting vaccinated.

"I think that's going to be very hard to enforce because people are going to think this vaccine is like a pass, like you're going to have a passport that you're vaccinated," she said. "But we don't have the clinical trials to show that people who are vaccinated are not shedding the virus. They might not be getting sick, but they might still be shedding if they got it. So they still have to wear their mask."

The U.K. granted approval to the Pfizer vaccine last week following a swifter process that largely relied on Pfizer and BioNTech's own analysis. Dr. Anthony Fauci, the top U.S. infectious disease official, criticized the U.K. approval process as "rushed," then later apologized and said he had not meant to imply any "sloppiness" by the British health officials.