WASHINGTON - 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.

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 Joe 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 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.