The Food and Drug Administration on Wednesday authorized a Pfizer-BioNTech coronavirus booster shot for people 65 and older and adults at risk of severe illness, an effort to bolster protection for the most vulnerable Americans against the highly transmissible delta variant of the virus.

In addition to older Americans, boosters should be made available to people 18 through 64 years of age at high risk of severe illness from the coronavirus and those “whose frequent institutional or occupational exposure” to the virus puts them at high risk of serious complications from the disease caused by the virus, the agency said.

The agency said the extra dose of the Pfizer-BioNTech vaccine should be administered six months after its standard two-shot regimen.

The FDA, in issuing the emergency clearance, took an approach similar to what was recommended Friday by the agency’s outside panel of vaccine experts. But the agency interpreted the advisory panel’s guidance broadly to cover a larger swath of people.

“The FDA considered the committee’s input and conducted its own thorough review of the submitted data to reach today’s decision,” Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in a statement. “We will continue to analyze data submitted to the FDA pertaining to the use of booster doses of COVID-19 vaccines and we will make further decisions as appropriate based on the data.”

The FDA action is not the final step before the booster is made available. The Centers for Disease Control and Prevention and its advisers still must recommend in detail who should receive the shots.

Expert advisers to the Food and Drug Administration on Sept. 17 recommended a coronavirus vaccine booster to people 65 and older and those at high risk. (The Washington Post)

“Today’s action demonstrates that science and the currently available data continue to guide the FDA’s decision-making for covid-19 vaccines during this pandemic,” acting FDA commissioner Janet Woodcock said in a statement.

“After considering the totality of the available scientific evidence and the deliberations of our advisory committee of independent, external experts, the FDA amended the [emergency use authorization] for the Pfizer-BioNTech COVID-19 Vaccine to allow for a booster dose in certain populations such as health care workers, teachers and day care staff, grocery workers and those in homeless shelters or prisons, among others,” Woodcock said.

The CDC’s Advisory Committee on Immunization Practices met Wednesday to hear data about the safety and effectiveness of a Pfizer-BioNTech booster, and it is expected to make a recommendation for its use when the committee meets Thursday. The decision will hinge in large part on which groups the advisers believe are most at risk of becoming seriously ill from covid-19, the illness caused by the coronavirus.

The recommendation from the CDC advisory committee will almost certainly receive a quick endorsement from agency Director Rochelle Walensky. The shots are expected to be available as soon as this week at pharmacies and some doctor’s offices.

The FDA decision Wednesday will help frame the recommendation by the CDC advisory panel on who should get the boosters. The challenge for the CDC advisers “is to make a recommendation that is appropriate from the public health perspective and easy to communicate,” said William Schaffner, an infectious-disease professor at Vanderbilt University School of Medicine and a liaison member of the vaccine advisory panel.

The CDC advisers, in previous meetings, have expressed a strong inclination to prioritize boosters for groups who were among the first to be vaccinated, including front-line health-care workers, people in nursing homes and then the elderly.

Several CDC advisers reiterated Wednesday that the key goal of booster shots should be the prevention of serious disease, not prevention of mild infection.

But Beth Bell, a global health expert at the University of Washington, said the panel should consider the importance of protecting health-care employees from even mild infections because those illnesses would require time off work.

Bell acknowledged that the panel will not be able to address many outstanding questions about boosters, such as the ability to use a Pfizer booster for people who have been vaccinated with the Moderna or Johnson & Johnson products.

“I think it’s extremely important for us to be keeping in mind that … these are interim recommendations, and there may be data forthcoming in the near future which might change those recommendations,” she said Wednesday.

The federal action on boosters this week pertains only to Pfizer-BioNTech. Federal officials have suggested that clearance of booster shots for the Moderna and Johnson & Johnson vaccines could happen in a few weeks. An FDA official participating in the CDC advisers meeting, Doran Fink, said the FDA has no data on the safety and efficacy of giving a Pfizer booster to someone who received a different vaccine.

But limiting action to the Pfizer product poses immediate practical challenges for front-line immunization and health workers, issues that surfaced repeatedly during the CDC vaccine advisory committee meeting Wednesday. Nursing home residents and employees, for example, have received Pfizer and Moderna shots.

“In North Dakota, the vast majority [of long-term-care facilities] had a mixture of people that needed both Moderna and Pfizer,” said Molly Howell, director of North Dakota’s immunization program and a liaison member to the CDC panel. “I don’t know that it’s realistic to keep going back with different brands.”

In some states, hospital systems already have been considering or moving forward with boosters before the FDA and CDC decisions.

“Our hospitals here in Alaska and many hospitals in the country are really facing unprecedented choices and decisions about care and trying to weigh the risk-benefit of things like a booster shot versus, you know, waiting,” Anne Zink, Alaska’s chief medical officer, said in a recent interview.

Federal health officials have already authorized a third shot of Pfizer and Moderna vaccines for people with weakened immune systems, such as cancer patients and organ-transplant recipients.

Some Americans, regardless of their health status, have received extra doses by showing up at pharmacies, sometimes on the advice of their clinicians. Consumers are not required to get a prescription or a letter from their doctors for the third shot. More than 2.3 million people have received an additional dose since an extra shot was authorized for people with frail immune systems, according to the CDC.

Limited data from Israel and from one of the U.S. coronavirus vaccine monitoring systems shows there were fewer side effects after a third dose of mRNA vaccines compared with the second dose, according to a presentation to the CDC advisers by Helen Keipp Talbot, an infectious-disease expert at Vanderbilt University who chairs the advisers’ safety work group.

The Biden administration announced in mid-August that it hoped to make boosters for all three vaccines widely available beginning this week, pending sign-offs from the FDA and the CDC. But that announcement set off weeks of controversy as angry scientists complained the White House had gotten ahead of the agencies’ vaccine experts.

Ultimately, the heads of the CDC and the FDA told the White House that only the Pfizer-BioNTech booster could be cleared by this week. And on Friday, the FDA advisers declined to greenlight the Pfizer-BioNTech booster for all adults, saying there was a lack of robust safety data for younger adults. Several also argued that the two-shot regimen remains highly protective against serious illness, even though there are signs the efficacy of the vaccines may be waning somewhat against mild infections.

Booster shots have emerged as a hot topic as the delta variant has spread havoc in some areas, with intensive care units filling up with unvaccinated patients and a few hospitals rationing care. The pandemic’s death toll exceeds the 675,000 Americans killed by the 1918 influenza pandemic, according to data tracked by The Washington Post — although the population of the United States today is three times as large as in 1918.

At the same time, critics have pressed the Biden administration to wage a more vigorous campaign to get shots to countries that have not had access to vaccines, and for wealthier countries to forswear boosters until more people receive first shots. The administration, in an effort to show it can provide boosters to Americans while offering shots to other nations, announced Wednesday that it has bought an additional 500 million doses to send overseas.

Carolyn Y. Johnson contributed to this report.