The Pfizer-BioNTech coronaviruses vaccine can be given to another group of people, 16- and 17-year-olds, at least six months after they received their second shot. The move clears the way for millions of teenagers to get another shot.
The decision by the FDA came as it was expected, as initial laboratory tests suggested that the new fast-spreading variant, Omicron, dulls the power of two doses of the Pfizer vaccine.
Everyone 18 and older can get a booster six months after getting a second shot of the Pfizer or Moderna vaccine or two months after a Johnson & Johnson shot. More than 50 million Americans have gotten the additional shots.
Pfizer was allowed to cover the younger age group on an emergency basis. The Moderna and Johnson & Johnson vaccines are only for adults.
According to new evidence, the vaccine effectiveness against Covid-19 is waning after the second dose for all adults and for those in the 16- and 17-year-old age group. He said that a booster would help provide continued protection against Covid-19.
Dr. Marks did not address whether the vaccine effectiveness was also going down for kids who became eligible for the vaccine in May.
Regulators reviewed immune response data from adults that showed more robust levels a month after booster doses were given. They said the analysis showed the effectiveness of a booster shot.
Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, signed off on the move, saying the agency was encouraging adolescents to get a booster dose.
She said initial data suggests that Covid-19 boosters help broaden and strengthen the protection against Omicron and other variant.
Before meeting his Covid-19 response team at the White House, President Biden called the development "even more great news from the F.D.A. and C.D.C." He said that if you got vaccine six months ago, you should get your booster now.
The F.D.A. and the C.D.C. acted without the input of their expert advisory panels. Before the agencies act, the committees discuss whether to authorize or expand eligibility for shots in public meetings.
Dr. Jesse L. Goodman, a former chief scientist at the F.D.A., said that more public discussion about boosters would have been helpful given the spirited debate this fall among the agency's regulators and outside advisers.
Dr. Goodman said that transparency lets people make more informed decisions. He said that recent data from Israel and other countries that suggest minimal serious side effects in younger people might help allay some parents concerns about a third dose for older male teenagers.
Public health experts are now in favor of boosters in the face of the Omicron variant, and some who previously opposed them now support a broad campaign. The expert committee of the F.D.A. recommended rejecting Pfizer-BioNTech's request to clear a booster shot for 16- and 17-year-olds because of insufficient data about a rare heart condition tied to the Pfizer-Bio.
Myocarditis, inflammation of the heart muscle, can be caused by the virus. Federal scientists say that cases tend to be mild.
The benefits of a booster dose in warding off the virus outweigh the risks of myocarditis, according to regulators.
More than two-thirds of 16- and 17-year-olds have received at least one dose of Pfizer's vaccine, according to federal data. More than 4 million people have received two doses. Three million people got their second shot at least six months ago and are eligible for a third shot this month.
Aggressive booster campaigns in wealthier countries could make it harder for people to get vaccines.
Richard Mihigo, the World Health Organization's vaccine program in Africa, said that high-income countries are giving more booster doses than they are giving in developing countries.
Biden administration officials say boosters shots don't limit donations. The United States has pledged more to countries in need than any other country. Nearly half a billion doses will be delivered by the end of the year, officials said. Some African countries want their shipments held up because they have too much supply.
Everyone who is eligible for a booster should get one because Omicron contains dozens of new genes.
Jen Psaki, the White House press secretary, said on Thursday that the government's health and medical experts would continue to assess the situation.
The F.D.A.'s latest booster authorization comes after preliminary results from Pfizer and BioNTech showed that people who had received two doses of the vaccine had lower levels of Omicron in their blood. The companies said that the results suggest that two doses may not be enough to protect against infections.
The companies said that the level of antibodies working to counteract the Omicron variant was comparable to that of the original variant after two doses. The immune system has many defenses. Severe cases of Covid-19 are expected to be protected by virus- attacking cells spurred by an initial two doses.
The reporting was contributed by Lynsey Chutel.
