WASHINGTON - A two-shot coronavirus vaccine developed by Pfizer and German firm BioNTech appears to be on the cusp of regulatory clearance, after a 53-page review published Tuesday confirmed that the vaccine meets the standard for emergency use authorization. 

The Food and Drug Administration's review, the first hint of how its career scientists are approaching one of the most momentous decisions in the agency's history, arrived during a week that could mark a turning point in the pandemic. In Britain, elderly people and nursing home caregivers began receiving shots of Pfizer's vaccine, even as hospitals in the United States prepared to receive shipments if the Pfizer vaccine gets the greenlight. The first peer-reviewed publication of results from the University of Oxford and AstraZeneca vaccine candidate fueled that company's push toward approval in some countries.

But dueling events by the incoming Biden administration and the outgoing Trump administration highlighted the dissonance between historic success in vaccine development and the growing crisis around the country as the United States surpassed 15 million confirmed cases, just five days after reaching 14 million.

At Joe Biden's event in Wilmington, Del., the president-elect unveiled a 100-day plan to rein in the coronavirus and acknowledged the difficult struggle ahead. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, appeared by video and endorsed a goal of near-universal mask-wearing this winter and spring. In contrast, the mood at the White House was jubilant, the event opening with a video seeming to mock scientists who had doubted the ambitious timeline for vaccine development, including Fauci.

"Very few people thought that this was possible," President Donald Trump said when he took the stage.

A weekly White House coronavirus task force report to governors struck a grim tone, noting that the current surge is the "most rapid increase in cases; the widest spread of intense transmission, with more than 2,000 counties in COVID-19 red zones; and the longest duration of rapid increase." It stresses that vaccines under review "will not substantially reduce viral spread, hospitalizations, or fatalities" until 100 million high-risk Americans are fully immunized, which will take until late spring.

Although distributing a vaccine promises to be a logistically complex and time-consuming proposition, the FDA briefing document is the surest sign so far that the Pfizer vaccine is on track to be cleared for use in the United States. The independent review provides the greatest transparency yet about data so far announced only by news release.

FDA scientists found the vaccine was 95% effective at preventing illness after two shots spaced three weeks apart. They identified a promising signal that the vaccine appeared to provide a level of protection even after a single shot, meaning vaccinations could begin to have an impact sooner after immunization than many had expected.

An advisory committee to the FDA is scheduled to hold an all-day virtual meeting to recommend to the agency whether the vaccine should be authorized for use in people ages 16 and up. A decision, informed but not dictated by that debate, is expected to come within days of the meeting. The first doses are expected to be shipped within hours of a decision, with a target of 6.4 million doses distributed in the first week.

Biden said a "herculean task" lies ahead to distribute vaccines but pledged to pursue "the most efficient mass vaccination plan in U.S. history," resulting in 100 million people receiving shots in 100 days. He said it would be essential for Congress to provide money for the effort.

The document illustrates that the vaccine has clearly surpassed the minimum threshold of being at least 50% effective. Reviewers found that the two months of follow-up on 38,000 participants in the trial provide evidence of "a favorable safety profile, with no specific safety concerns identified that would preclude issuance of an [emergency authorization]." The side effects included sore arms, fatigue, headaches, muscle pain and chills that resolved within a few days.

The finding most likely to provoke debate shows that the first shot protected against COVID-19, the disease caused by the virus, and was estimated to be 52% effective in the three weeks between the two shots. But the FDA noted there wasn't enough data to draw firm conclusions.

Government officials have said they plan to hold back the second shot when distributing vaccines, to ensure sufficient supply of the booster shot that provides complete protection. But the signal of early protection from a single dose has led some experts to suggest that may not be the best way to use limited doses amid surging cases.

"This is a crisis," Scott Gottlieb, former FDA commissioner and a board member at Pfizer, told CBS's "Face the Nation" on Sunday. "We need to get as many vaccines in arms as possible in my view. And that means pushing out all the available supply or most of it."

Others point out that the signal of protection comes from just the three weeks between participants' first and second shot, and the study wasn't designed to answer the question of whether a single shot can offer protection. What if immunity wanes quickly?

"Stick to what you know, stick to what you've proven - don't sort of make it up as you go along," said Paul Offit, a vaccine expert at Children's Hospital of Philadelphia and a member of the FDA advisory committee set to consider the vaccine Thursday.

The vaccine was tested in about 44,000 people, half of whom received two doses of the real vaccine three weeks apart, and half of whom received shots of saline. People then went about their normal lives and were exposed to the virus in their communities. There were 170 cases of COVID-19 in the study. Only eight of those cases were in the group that received the vaccine, an overwhelming indication of efficacy and well above the minimum threshold the agency laid out this summer.

One of the biggest questions about the coronavirus vaccines is whether they work equally well in all people, particularly those at highest risk of severe illness, including older people, those with underlying medical conditions and minority groups. The vaccine trials sought to recruit diverse populations.

The FDA review found the efficacy of the vaccine was "uniformly high" across subgroups, except for participants identifying as multiracial and those who had been infected with the coronavirus before enrollment. It said there were too few COVID-19 cases in those subgroups to interpret efficacy.

Another major question about the vaccines has been whether they can prevent both mild and the most devastating, severe cases of disease. There were 10 cases of severe illness in the trial, with nine of those in the placebo group.

The small number of cases "limits the overall conclusions that can be drawn; however, the case split does suggest protection from severe COVID-19 disease," reviewers wrote.

Side effects from the vaccine were common, but short-lived. Many participants experienced soreness at the injection site, fatigue and headaches. There were four cases of Bell's palsy, a temporary paralysis of facial muscles, in the group that received the vaccine, but reviewers did not suspect a link to the vaccine.

One area of debate among outside advisers to the agency this week could be the length of follow-up on a vaccine technology that has never been used in an approved product.

The study could not determine whether people who have previously been infected with COVID-19 benefited from vaccination, but the FDA concluded that the limited data "do suggest that previously infected individuals can be at risk of COVID-19 [reinfection] and could benefit from vaccination."

Major questions that remain unanswered about the vaccine include how long the protection will last and whether the vaccines prevent people who are infected despite vaccination from spreading the disease.

The trial showed no evidence that immunity subsided during the two months after the second dose, but more data will need to be gathered to assess whether protection tapers off. The reviewers called for further study of whether people, particularly those with asymptomatic infections, spread the disease. Epidemiologists have worried about a worst-case scenario in which people who are vaccinated stop wearing masks but may become asymptomatically infected and spread the disease.

"It is possible that if efficacy against asymptomatic infection were lower than efficacy against symptomatic infection, asymptomatic cases in combination with reduced mask-wearing and social distancing could result in significant continued transmission," the review states.

The Pfizer vaccine is the first of two vaccines to be considered by regulators this month based on a novel technology that uses snippets of genetic material to teach the body's own cells to build the spiky protein that coronaviruses use to latch onto cells. The immune system, exposed to the spike, can then learn to recognize and block the real virus.

A shot from Moderna, a biotech firm with a coronavirus vaccine based on the same technology and backed by similar results, is expected to be debated by the same advisory committee next week. In anticipation of both receiving clearance, government officials have projected having 40 million doses - enough to inoculate 20 million people - from both companies by the end of the year.

Pfizer will deliver the balance of the 100 million doses the United States has purchased in the first few months of next year, enough for 50 million people. Government officials have projected a cadence of vaccination that could accelerate when more vaccines become available, as manufacturing capacity increases and as other companies' vaccines may gain regulatory authorization.

But already, questions are emerging about the long-term supply. The United States approached Pfizer to buy another 100 million doses this past weekend, but was informed the supply may not be available until late June, The Washington Post reported. That raises questions about whether the U.S. goal to create enough vaccine for the entire population by the summer could slip.