WASHINGTON — Federal regulators on Thursday authorized booster shots of the Pfizer-BioNTech coronavirus vaccine for another swath of the population, 16- and 17-year-olds, at least six months after they received their second shot of that vaccine. The move clears the way for several million teenagers to receive an additional shot.

The decision by the Food and Drug Administration, which was expected, comes as initial laboratory tests have suggested that the new fast-spreading variant, Omicron, dulls the power of two doses of the Pfizer vaccine.

Everyone 18 and older has been eligible since Nov. 19 to receive a booster six months after getting a second shot of the Pfizer or Moderna vaccine, or two months after a Johnson & Johnson shot. Nearly 50 million Americans — a quarter of those fully vaccinated — have gotten the additional shots.

The F.D.A. broadened Pfizer’s authorization to cover the younger age group on an emergency basis. The Moderna and Johnson & Johnson vaccines remain authorized only for adults.

“Since we first authorized the vaccine, new evidence indicates that vaccine effectiveness against Covid-19 is waning after the second dose of the vaccine for all adults and for those in the 16- and 17-year-old age group,” said Dr. Peter Marks, who oversees the F.D.A.’s vaccine division. A booster, he added, “will help provide continued protection against Covid-19 in this and older age groups.”

Dr. Marks did not address whether vaccine effectiveness was also ebbing for 12- to 15-year-olds, who became eligible for the Pfizer vaccine in May.

Regulators reviewed immune response data from adults that showed more robust antibody levels a month after booster doses were given. They said the analysis demonstrated the effectiveness of a booster shot for adolescents.

Dr. Rochelle P. Walensky, the Centers for Disease Control and Prevention director, promptly signed off on the move, saying the agency was encouraging adolescents to receive a booster dose.

“Although we don’t have all the answers on the Omicron variant, initial data suggests that Covid-19 boosters help broaden and strengthen the protection against Omicron and other variants,” she said in a statement.

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 added, “I say it to all of you: If you got vaccinated six months ago, get your booster right away.”

The F.D.A. and the C.D.C., which sets vaccine policy for the federal government, acted without input from their independent expert advisory panels. The committees have typically discussed whether to authorize or expand eligibility for shots in public meetings before the agencies act.

Dr. Jesse L. Goodman, a former chief scientist at the F.D.A., said that given the spirited debate about boosters this fall among the agency’s regulators and outside advisers, more public discussion would have been helpful.

“Transparency lets people make more informed decisions,” Dr. Goodman said. For instance, he said, recent data from Israel and elsewhere that suggests minimal serious side effects in younger, boosted people might help allay some parents’ concerns about a third dose for older male teenagers.

Support for boosters has been growing among public health experts in the face of the Omicron variant; some who previously opposed them now support a broad campaign. The F.D.A.’s expert committee recommended rejecting Pfizer-BioNTech’s request in September to clear a booster shot for 16- and 17-year-olds, partly because of concerns over what the panel saw as insufficient data about a rare heart condition tied to the Pfizer-BioNTech and Moderna shots, especially in younger men.

That condition, myocarditis, or inflammation of the heart muscle, can also be caused by the virus. Federal scientists have said that cases tend to be mild and resolve quickly.

Regulators said on Thursday that they had determined in reviewing Pfizer-BioNTech’s application that the benefits of a booster dose in warding off the virus and its health consequences outweighed the risks of myocarditis.

According to federal data, about 5.5 million 16- and 17-year-olds — two-thirds of that age group — have received at least one dose of Pfizer’s vaccine. More than 4.7 million have received two doses. Roughly three million got their second shot at least six months ago and would be eligible for a third shot this month.

The World Health Organization on Thursday again expressed concern that aggressive booster campaigns in wealthier countries could further exacerbate global disparities in access to vaccines.

“If we looked at the data today, even before Omicron, we are seeing high-income countries administering more booster doses than even vaccines that are being given in developing countries,” Richard Mihigo, the coordinator for the World Health Organization’s vaccine program in Africa, said at a news conference.

Biden administration officials argue that boosters shots do not limit donations. According to the White House, the United States has pledged 1.2 billion doses to countries in need — more than all other countries combined. By roughly three months from now, nearly half a billion doses will have been delivered, officials said. Some African countries have asked that shipments be held up because they have more supply than they can manage.

Federal health officials say that because Omicron contains dozens of mutations never seen before, it is all the more important that everyone eligible for a booster gets one.

Asked whether a fourth shot might be necessary, 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 follows preliminary results that Pfizer and BioNTech reported on Wednesday showing that tests of blood from people who had received two doses of the companies’ vaccine contained much lower levels of antibodies against Omicron than against an earlier version of the virus. Antibodies are the immune system’s first line of defense against the virus, and the results suggest that two doses may not be sufficient to protect against infection, the companies said.

With a booster, the level of antibodies working to neutralize the Omicron variant was comparable to that combating the original variant after two doses, the companies said. Antibodies are just one measure of the immune system’s defenses. Virus-attacking cells spurred by an initial two doses are still expected to offer substantial protection against severe cases of Covid-19.