WASHINGTON — The Food and Drug Administration on Friday authorized booster shots of both the Pfizer-BioNTech and Moderna vaccines for everyone 18 and older, opening up eligibility to tens of millions more fully vaccinated adults.

Moderna and Pfizer both announced that regulators had broadened the authorization for their booster shots.

The move simplifies eligibility, fulfills a pledge by President Biden to offer the shots to every American adult and formally allows a practice already in place in at least 10 states. Fearful that waning protection and the onset of winter will set off a wave of breakthrough infections, a growing number of governors had already offered boosters to everyone 18 and older ahead of the holidays.

Dr. Anthony S. Fauci, the federal government’s top infectious disease expert, has argued relentlessly over the past month for booster shots for all adults, a position shared by most of Mr. Biden’s other health advisers. Dr. Fauci has said that a dip in antibody levels in fully vaccinated people was a clear sign that booster shots were needed. Public health experts who argue that healthy younger adults do not need them, he has said, are ignoring the risks of symptomatic Covid-19.

“Enough is enough. Let’s get moving on here,” he said at an event Wednesday night. “We know what the data are.”

If the Centers for Disease Control and Prevention agrees, all adults who received a second shot of either Pfizer or Moderna at least six months ago will likely be eligible for a booster shot by the weekend. A meeting of the agency’s outside advisers is scheduled for Friday.

At a White House briefing on Wednesday, Dr. Rochelle Walensky, the C.D.C. director, promised that the agency would “quickly review the safety and effectiveness data and make recommendations as soon as we hear from F.D.A.”

The F.D.A.’s action came after months of fierce debate within the administration and the scientific community about who needed booster shots, and when. Some outside advisers to the F.D.A. and C.D.C. repeatedly expressed discomfort with how quickly the administration was moving to offer the shots. Critics said the administration’s booster campaign was motivated by practical or political reasons more than scientific ones, and that federal regulators were analyzing safety and efficacy data on the fly.

“The evidence isn’t there that a large rollout of boosters is really going to have that much impact on the epidemic,” said Ira M. Longini Jr., a vaccine expert and professor of biostatistics at the University of Florida. He said that booster doses could increase someone’s protection at least temporarily, but would do little to halt transmission of the virus, which is being driven by the unvaccinated. For a jittery public, he said, booster shots might feel like a quick and easy way to hold the virus at bay. But he repeated the administration’s own oft-stated position that convincing the unvaccinated to get the shots must remain the top priority.

Other public health experts have argued that the government needed to offer boosters to all adults to eliminate confusion. Complex eligibility rules coupled with the government’s recent decision to let people choose among all three vaccines for their booster has left the public somewhat befuddled, they say.

“This decision by F.D.A. is overdue,” said Dr. Elizabeth McNally, director of the Center for Genetic Medicine at Northwestern University’s Feinberg School of Medicine.

“Many people had trouble understanding whether they should or shouldn’t get boosters,” she said in a statement. “This message is much clearer — get a booster!”

Until now, those eligible for boosters included people 65 and older, residents of long-term care facilities, people with underlying medical conditions, and those whose jobs or institutional living conditions heightened their risk.

All recipients of Johnson & Johnson’s one-shot vaccine have already been cleared to get a booster shot at least two months after their injection.

Jason L. Schwartz, an associate professor of health policy at the Yale School of Public Health, said the latest authorizations were “a recognition that the current approach to booster recommendations just isn’t working.”

“It’s so confusing that I think the public has sort of shrugged at the importance of boosters,” he said. “And the groups for whom it’s really important to get boosters — the older individuals, the long-term care residents, folks with medical conditions — aren’t getting boosters at the rates they should. It’s time for a reset.”

The booster rollout is expanding even as public health experts continue to argue over whether young, healthy adults need added protection. The argument is stronger for recipients of Pfizer’s vaccine than for recipients of Moderna’s, which has proved significantly more potent.