In a separate move aimed at answering longer-term questions about booster strategies, the FDA plans to hold a meeting of its outside advisers in early April to consider whether there should be an effort in October or November, perhaps in conjunction with the annual influenza vaccine campaign, to encourage some or all adults to get additional boosters. The panel could also discuss whether the shots should be the same formula as current vaccines or retooled to counter new variants, according to a federal official who spoke on the condition of anonymity to discuss the plans.

Increasingly, some Biden administration officials have signaled they believe adults of all ages might need a second booster, because of the lack of durability of the two-shot mRNA vaccines made by Pfizer-BioNTech and Moderna.

Budget problems also could complicate discussions about providing boosters for everyone later this year. In a call with reporters Tuesday, senior administration officials said they do not have the money for a potential fourth shot for all Americans following last week’s collapse of a $15.6 billion plan to fund vaccines and other coronavirus countermeasures.

Albert Bourla, chief executive of Pfizer, said at a Washington Post Live event last week that a fourth shot would be needed because immunity wanes.

He told CBS’s “Face the Nation” in an interview that aired Sunday that the protection provided by the first booster is “actually quite good for hospitalizations and deaths. It’s not that good against infections.” Pfizer and BioNTech are also working on a vaccine targeting all variants and aimed at providing protection for at least a year.

The FDA declined to comment. Typically, after the FDA authorizes a vaccine, the Centers for Disease Control and Prevention’s vaccine advisory panel weighs in on a recommendation, which must get approved by the CDC director before it becomes official policy.

“The proof in the pudding is how long protection following a third boost lasts — five, six, seven, eight months out,” Fauci said. “If it goes down, then you make your decision about whether you’re going to boost based on the clinical data.”

Biden administration officials are looking for clarity on two fronts. They want to see the extent to which immunity wanes after the first booster shot and the efficacy of a second booster.

Pfizer cited data from the Israeli Health Ministry that followed 1.1 million people 60 and older who were eligible for a fourth shot. 

That study, published on a preprint server before peer review, tracked infections and hospitalizations in Israel in the second half of January, after a fourth dose began to be offered. Rates of infection were lower for those who received a fourth dose compared with those who had received only three shots. The rates of severe illness were substantially lower in people who received a fourth shot. It is not yet clear how long that protection would last.

“Giving the fourth dose to individuals who were at risk to develop severe disease has been instrumental in limiting the burden on hospitals in Israel during the fast and wide-spreading Omicron surge,” the researchers concluded.

Pfizer also cited a separate preprint study from Israel that tested a fourth shot in health-care workers, which found a mixed picture. A fourth shot of the Pfizer-BioNTech or Moderna shot increased virus-fighting antibodies but was not very effective at preventing less severe or asymptomatic infections. That suggests that as a longer-term strategy — and for people who are not at high risk of severe disease — a fourth shot may not be the ideal way to increase immunity. Infections among vaccinated people were common, and people had large amounts of virus in their noses, suggesting they could infect others.

Pfizer and BioNTech are formally testing a fourth shot in a clinical trial that began in January. In one group of 600 fully vaccinated and boosted people, they are comparing a version of their vaccine fine-tuned to fight the omicron variant with a fourth shot of the regular vaccine.

Moderna is also testing additional booster doses, including an omicron-specific shot and one that combines vaccine targeting omicron with its original shot. The hope is that a combination could produce a longer-lasting immune response. Moderna did not immediately respond to questions about whether the company would seek authorization for a fourth shot of its current vaccine, but Moderna President Stephen Hoge told Insider this week that he thought a fourth shot would be beneficial in older adults — the same group for which Pfizer is seeking clearance of a fourth shot.

“For those who are immune-compromised, those who are older adults, over the age of 50 or at least 65, we want to strongly recommend and encourage [a fourth shot], the same way we do with flu vaccines,” Hoge told Insider.

The American public — and even experts — have been sharply divided on coronavirus vaccines and boosters, with the United States falling far short of vaccination rates seen in many other countries. Pfizer’s announcement that a third shot would be needed last summer helped sow some of that confusion. The company got pushback from government officials who had not yet been persuaded one was needed, although they later called for boosters.

Now, with some studies showing that vaccine effectiveness has ebbed to some extent in the face of the highly transmissible omicron variant, some older people have been clamoring for a second booster — or getting one even before it is officially authorized.

Concerned about reports that booster effectiveness wanes, Dewey Martin of Hampden, Maine, has made several attempts to get an additional shot. He visited two pharmacies, where employees told him only the immunocompromised are eligible, and used an online patient portal to ask his doctor about getting one.

“Even though I’m 72, I’m pretty healthy, and I don’t think anything serious would happen to me,” Martin said. “But I don’t want to take that chance, especially since the shots didn’t affect me at all.” He said he was frustrated the approval has not come sooner: “Why not let us have it?”

Other individuals, of all ages, appear to have little interest in the shots, especially as the omicron threat fades and infections and hospitalizations plunge.

Officials in the Biden administration and the public health community are keeping a close eye on a rise in coronavirus cases in Europe and the proliferation of the omicron variant BA.2. Some experts are worried cases could increase in the United States following the relaxation of mask requirements as BA.2, which is more transmissible than the original version of omicron, becomes dominant.

“It is something that you have to watch very closely,” said the federal official who spoke on the condition of anonymity. “We don’t want to panic. On the other hand, we don’t want to be tone-deaf or blind to things. More often than not, when we have seen things happen in Europe and elsewhere in the globe, we wind up seeing them here.” The official said cases are rising overseas as restrictions such as mask requirements are easing there.

A study published last month in the CDC’s Morbidity and Mortality Weekly Report suggested boosters of Pfizer-BioNTech and Moderna vaccines lost some effectiveness after four months but still offered strong protection against severe illness and hospitalization. The study said the vaccines appeared to be more effective against the earlier delta variant than the omicron variant.

In January, Israel began offering a fourth dose of the Pfizer-BioNTech vaccine to people 60 and older and medical workers. In France, second boosters are available for everyone 80 and older and people who are immunocompromised or have a long-term illness. Chile and Germany also recommend fourth shots for high-risk groups.

In the United States, four shots of the vaccine are already authorized for people who have moderately or severely compromised immune systems, which hinder an effective response to the vaccine.

According to the CDC, about 65 percent of the U.S. population is fully vaccinated, but only 44 percent of those individuals have received boosters.

Brittany Shammas and Lena H. Sun contributed to this report.