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.

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.

The agency said the expansion was justified by currently available clinical trial data as well as real-world evidence. In a statement, Dr. Peter Marks, who leads the F.D.A. division that regulates vaccines, added: “Streamlining the eligibility criteria and making booster doses available to all individuals 18 years of age and older will also help to eliminate confusion about who may receive a booster dose and ensure booster doses are available to all who may need one.”

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. 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.”

Scientific advisers to the Centers for Disease Control and Prevention on Friday unanimously endorsed booster shots of the Pfizer-BioNTech and Moderna coronavirus vaccines for all adults.

If Dr. Rochelle Walensky, the agency’s director, formally accepts the recommendation, as she typically does, those who received a second shot of either Pfizer or Moderna at least six months ago will most likely be able to get a booster shot by the weekend.

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.