Pfizer and its German partner BioNTech are expected to seek authorization for their coronavirus vaccine booster shot for anyone 18 and older, a move that could increase booster rates at a critical moment in the pandemic, according to three officials familiar with the situation.

The request, which may be filed as soon as this week, is likely to win the backing of the Food and Drug Administration, said the individuals, who spoke on the condition of anonymity because they were not authorized to discuss the issue. That would essentially fulfill the Biden administration’s booster-for-all-adults goal, announced last August amid concerns about waning vaccine protection.

Pfizer spokesman Kit Longley declined to confirm the company’s plans, saying he had no update on boosters.

While the surge caused by the delta variant appears to be subsiding, some health experts are concerned that infections and deaths might be plateauing at a high level. Cases still exceed 70,000 a day, and deaths from covid-19, the illness caused by the virus, total more than 1,000 daily, according to The Washington Post covid-19 tracker.

Almost a year after their parents and grandparents became eligible, children ages 5 to 11 are now eligible for the two-dose coronavirus vaccine from Pfizer. (Luis Velarde/The Washington Post)

In addition, state and federal health officials are casting an anxious eye on states including Minnesota, Colorado, Arizona and Vermont, where cases have been rising. They’re also worried about increased viral spread during a busy holiday season, with large numbers of people traveling and celebrating indoors.

In mid-August, President Biden announced plans to make boosters available to all adults beginning the week of Sept. 20. But the administration backed off after receiving sharp criticism from many scientists and public health experts — including several of its own advisers — who said there was little evidence that young, healthy people needed the extra shot, especially when much of the world remained unvaccinated.

Since then, however, additional data has underscored the shots’ safety and shown more definitively that their protection does wane over several months.

“In August, I thought it was extremely premature to offer boosters to everyone,” partly because of a lack of data, said Jeanne Marrazzo, an infectious-diseases physician at the University of Alabama at Birmingham. “The idea of rolling out boosters, while we were trying to increase our primary vaccination rates and dealing with the crushing delta wave, I felt was tone deaf.”

Now, Marrazzo said, additional data is showing that “durable immunity to the coronavirus is an elusive goal.” She said she has treated several cases of breakthrough infections among the vaccinated and while the patients typically do not end up in the hospital, “It’s not pretty. You don’t want to get a breakthrough infection.”

Authorizing boosters for all adults would be an explicit recognition of what is already occurring: People are getting access to the shots by saying they are in one of the recommended categories; pharmacies are using the honor system. Some experts say it’s time to get rid of complicated rules that require people to pretend that they are immunocompromised or work in a hospital.

Allowing the Pfizer-BioNTech booster to be given to anyone 18 and older also would give the green light to people who have eschewed boosters because they don’t fit into any specific category — or think they don’t. Some health officials say the multiple categories are confusing and have prevented some people from getting shots they are eligible for.

“There are people who will stick by the rules and others who have been saying for months, ‘Gimme, gimme, gimme,’ ” said John P. Moore, an immunologist at Weill Cornell Medicine.

Under the government current guidelines, people are eligible for Moderna and Pfizer BioNTech boosters if they are 65 and older or are at high risk of covid-19 because of underlying medical conditions or potential exposure at work or in their living situations. The shots are administered six months after the second dose of the two-shot vaccines. Boosters for the single-shot Johnson & Johnson vaccine are recommended for anyone 18 and older two months after the vaccine. The criteria reflects the lower protection afforded by that vaccine.

Top health officials also have given the go-ahead to “mix-and-match” booster shots to increase antibodies to fight the coronavirus. People can get any one of the three booster doses regardless of which shot they received first.

Under the recommendations, at least 179 million people, or 69 percent of the adult population are potentially eligible for boosters, according to administration officials. Other estimates put the number even higher.

“That horse has left the barn,” Eric Rubin, editor in chief of the New England Journal of Medicine and an adjunct professor at the Harvard T.H. Chan School of Public Health, said, referring to boosters. Rubin was one of a majority of FDA advisers who voted in September against a Pfizer-BioNTech request to offer boosters to everyone 16 and older.

Since then, Rubin said, millions more people have gotten boosters in the United States and abroad, and “I have not heard of any safety signals. That really does change things.” Moreover, emerging data from Israel shows “good-size decreases in severe disease among booster recipients,” he said.

Moore said the booster debate over the past several months has reflected divisions among scientists over the purpose of vaccines: Are they designed primarily to prevent severe disease and death or to prevent infections?

“Nobody is wrong on this,” he said. “It’s a genuine disagreement about the prevention of infection vs. the prevention of severe disease.”

Moore said the data justifying boosters has become more ample and persuasive in recent months but he still has reservations about a universal booster policy: “Do healthy 25-year-olds really need a third dose?”

A study published recently in Science showed that the effectiveness of all three vaccines in preventing infection dropped dramatically between March and September, as the delta variant surged. The vaccines’ protectiveness held up better against death, according to researchers who analyzed the records of almost 800,000 veterans.

Another study, published recently in the Lancet using records from Israel’s largest health maintenance organization, showed the impact of a booster shot for the Pfizer-BioNTech vaccine administered five months after the second shot. Compared with two doses, a third dose in people 12 and older was 93 percent effective in preventing hospitalizations and 81 percent effective in preventing death, the study found.

Israel now offers boosters to everyone over 12. Germany, which is battling what officials call a “fourth wave” in covid-related illnesses, said on Friday that it planned to begin offering boosters to everyone who had been vaccinated.

“Biologically, it makes sense” to need additional shots to get the greatest protection from the vaccines,” said Peter Chin-Hong, an infectious-disease physician at the University of California at San Francisco.

And with the holidays coming, he said, there is “some value” in young people getting a booster to protect older relatives. That’s especially true, he said, if the older person has an illness, such as some cancers, that makes it harder for the body to mount an immune response even with a vaccine.

The FDA is unlikely to take the Pfizer-BioNTech application to its advisory committee because the panel already has had extensive discussions about boosters, according to two officials who spoke on the condition of anonymity because they were not authorized to discuss the issue. If the agency authorizes the booster for all adults, CDC Director Rochelle Walensky would have to make updated recommendations for how to use the shot. It is not clear whether she would consult with outside experts on the Advisory Committee on Immunization Practices.

If that group were consulted, “there would be people who would ask for more data to indicate boosters were really needed in a younger, healthier age group,” said William Schaffner, an infectious-diseases physician at Vanderbilt University and a liaison member to the advisory group.

He predicted that some panel members would say, as they have previously, that the vaccines remain effective and there is not extensive data indicating waning protection against severe disease. But others might argue that public health officials “can’t get too nuanced here,” he said.

The boosters’ safety record “is very, very solid,” Schaffner said, adding that side effects appear limited to reactions at the injection site and “feeling crummy.” Physicians and other health-care providers already are discussing broadening eligibility, he said.

“The discussion has been, ‘Gee, why not just open it up to everyone and make it easier for us practitioners?’ ” Schaffner said. He added that if a patient told him she wanted to visit her grandparents and do everything possible to reduce the risk of spreading the virus, he would give the go-ahead for a booster, as long as the patient was six months past the last shot.

Wilbur Chen, a member of the CDC immunization advisory committee who earlier opposed a broad approach to boosters, said new data means “it’s very fair game for us to revisit this question.”

Chen, a professor of medicine at the University of Maryland School of Medicine, added, “You may end up with more people protected if you make it much simpler.”

Some experts have raised concerns about a link between the Pfizer-BioNTech and Moderna vaccines and myocarditis, a rare heart inflammation. But with substantial virus still circulating in the United States, Chin-Hong said, the benefits of a booster outweigh the risks, especially for those 18 and older. Scientists say the risk of myocarditis tends to be highest in 16- and 17-year-olds.

Moderna did not respond to a request for comment about whether the company will apply for authorization to make its booster available to all adults.