The Food and Drug Administration is aiming to authorize booster doses of Pfizer-BioNTech's coronaviruses vaccine for all adults as early as Thursday, a move that would expand the number of Americans eligible for additional shots by tens of millions, according to people familiar with the agency.
Data on the efficacy and safety of the booster dose will be the subject of a meeting on Friday. If both the F.D.A. and the C.D.C. sign off this week, they will have acted quickly after Pfizer asked for authorization of boosters for everyone 18 and older.
Any adult who received a second dose of the vaccine at least six months earlier would be eligible to get a booster as soon as this weekend. The F.D.A. is expected to rule without consulting its own expert panel, which has met frequently during the Pandemic to review vaccine data and make a recommendation ahead of a regulatory decision.
The broad booster authorization has been viewed as a foregone conclusion for weeks. Some state and local officials have begun rolling out similar policies ahead of the F.D.A. action, responding to persistent virus case counts, and the eagerness of many Americans to seek additional protection ahead of holiday gatherings.
New York City health officials are encouraging adults to look for boosters. Arkansas, California, Colorado and New Mexico have moved to expand access.
Many Americans have taken the matter into their own hands and sought out extra doses even if they did not officially qualify.
The F.D.A. reduced the initial request from Pfizer-BioNTech's to approve booster doses for all adults. It made a more limited population eligible, including those 65 and older, as well as adults with underlying medical conditions or those at risk because of their jobs.
According to some estimates, at least 30 to 40 percent of vaccine recipients are not eligible for booster eligibility.
More than 30 million people have already received additional shots, with the daily number often exceeding that of first shots given around the country. The Johnson & Johnson vaccine and vulnerable groups who received Moderna's two-dose shot were given booster doses in October.
The meeting on Friday is going to be briefer than recent meetings about Covid vaccines. One federal official familiar with the planning said that it is expected to be straightforward because of how far the nation has come. It would suggest a change of heart among public health experts since President Biden announced in August that he wanted to give booster shots to all adults.
Public health experts were against Mr. Biden's decision to start the campaign in late September, because they said he was rushing the federal scientists and regulators who were responsible for deciding whether the data supported such a move.
The effectiveness of the two-dose regimen has held up well according to some regulators and outside advisers. A large study in New York has shown that the protection gained from all three federally authorized vaccines lasts for a long time.
Some federal regulators and scientists say that a reduction in vaccine efficacy against milder Covid cases has put Americans at risk of harmful breakthrough infections as the Delta variant of the virus only.
Pfizer and BioNTech said last week that their request is based on data from a clinical trial in the United States and elsewhere. The vaccine's efficacy rate was restored to 95 percent after a third dose. It's not clear how long the protection from a booster shot will last.
Moderna is expected to submit its own request for the F.D.A. If it is broadly authorized, every adult can get the Pfizer booster.
The case for boosting in the United States is clear, as evidenced by the fact that Israel started its booster campaign in late July and now offers extra shots to everyone 12 and older. Dr. Fauci said at the event that the virus could be brought under control in the U.S. by the spring if everyone had boosters.
In the case of Covid vaccines, experts said that it was always expected to fade, as antibody protection is just one measure of how a vaccine confers protection. Keeping the levels high would require regular booster shots that would be difficult to sustain.
Getting the first vaccine dose is more important than boosting, experts say.
Several members of the F.D.A.'s expert committee said at a meeting last month that the agency should go ahead and lower the age eligibility for booster shots.
A study published in the scientific journal The Lancet in late October compared people who had received a booster dose in August or September with people who had only received two doses of the vaccine at least five months earlier.
Pfizer was expected to present to the F.D.A. in its request to expand eligibility after Israeli officials reported that third doses had not caused any significant safety concerns. The C.D.C. panel is likely to look at the data the company provides. Younger men who received two doses of Pfizer-BioNTech and Moderna vaccines have been found to have a higher risk of myocarditis, inflammation of the heart muscle.
The associate professor of microbiology and immunology at the University of Michigan said that she was initially skeptical about clearing booster shots for so many adults based on data from Israel.
She said thatCaution is always better. She said that clearing more adults to get boosters would make the messaging around booster shots more manageable.
The American booster campaign has beenhelter-skelter, according to Dr. Ofer Levy, director of the Precision Vaccines Program at Harvard's Boston Children's Hospital.
He said that each of us have our own personal psychology and risk tolerance and belief systems.
