The Food and Drug Administration (FDA) on Friday authorized booster shots of both the Pfizer-BioNTech and Moderna vaccines for ages 18 and older.

The agency said the expansion was justified by currently available clinical trial data as well as real-world proof. In a statement, Dr. Peter Marks, who leads the FDA 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.” 

A mounting number of governors had already offered boosters to everyone 18 and older ahead of the holidays, due to fears that declining protection and the onset of winter might start out a wave of breakthrough infections.

Dr. Anthony S. Fauci, the federal government’s top infectious disease expert, has been recently arguing ceaselessly for booster shots for all adults, a position most of Mr. Biden’s other health advisers agree on. Public health experts who argue that healthy younger adults do not need them, he has said, are ignoring the risks of symptomatic Covid-19.

The agency’s advisers are meeting on Friday to discuss who should be able to get the booster and will hear from Pfizer, Moderna, and C.D.C. staff. A vote is scheduled for 2:45 p.m.

Dr. Rochelle Walensky, the C.D.C. director, promised that the agency would do a quick review on the safety and effectiveness data and present recommendations as soon as we hear from FDA, at a White House briefing on Wednesday.

Critics have said practical or political reasons are behind the administration’s booster campaign, rather than scientific ones. Some said that federal regulators were quickly analyzing safety and efficacy data, while others are worried that moving to clear Pfizer and Moderna booster doses for all adults might inadvertently undermine the vaccines and suggest that two doses are not strongly protective.

Amid the booster debate, the question is what are vaccines supposed to do? Critics of the administration’s policy argue that despite some degree of waning protection, the vaccines are still fulfilling their task of protecting against severe disease and hospitalization.

In recent weeks, state after state has moved to allow booster shots for all adults, including Massachusetts, Rhode Island, Kansas, Kentucky, Maine, Vermont, Arkansas, California, Colorado, and New Mexico.

A host of other countries have taken the same approach, including Australia, Brazil, Canada, Germany, Hungary, Israel, Japan, Norway, and Saudi Arabia. Regulators for the European Union have authorized booster doses of both Moderna and Pfizer-BioNTech for all adults.