F.D.A. advisers must grapple with confounding research on booster shots.

Advisers to the Food and Drug Administration on Friday must confront a range of bewildering scientific questions before deciding whether to authorize coronavirus vaccine booster shots — and if so, for whom.

Three important pieces of research landed just this week. On Monday, in the journal The Lancet, an international team of scientists analyzed dozens of studies and concluded that boosters are not yet needed by the general population, and that the world would be better served by using vaccine doses to protect the billions of people who remain unvaccinated.

Two of the authors are vaccine experts at the F.D.A. itself, and both had already announced plans to resign over what they felt was undue pressure from the Biden administration to approve booster shots.

On Wednesday, scientists at the agency posted an assessment online hinting that they, too, are unconvinced that there’s enough evidence that boosters are needed. “Overall, data indicate that currently U.S.-licensed or authorized Covid-19 vaccines still afford protection against severe Covid-19 disease and death in the United States,” according to their executive summary.

White House officials have said they are particularly worried by data from Israel, where officials have said that vaccinated people are seeing waning immune responses and higher rates of infection. Alarmed by the rise in cases, Israeli officials offered third doses of the vaccine to everyone older than 12.

Researchers from Israel published early results from that rollout on Wednesday in the New England Journal of Medicine — but few outside scientists found the findings convincing.

The team collected data on the effects of booster shots from the health records of more than 1.1 million people over age 60. At least 12 days after the booster, rates of infection were elevenfold lower — and rates of severe disease nearly twentyfold lower — in those who received a booster compared with those who had received only two doses, the researchers found.

The results are unsurprising, experts said, and do not indicate long-term benefit.

“We have known for some time that the vaccines elicit less robust immune responses in the elderly,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center and a former adviser to the Biden administration. “Recommending additional doses of vaccine for the elderly isn’t controversial.”

Vaccination remains powerfully protective against severe illness and hospitalization in the vast majority of people in all of the studies published so far, experts said. But the vaccines do seem less potent against infections in people of all ages, particularly those exposed to the highly contagious Delta variant.

The cumulative data so far suggest that only older adults will need boosters — and maybe not even them. But White House officials have said that they do not want to wait for hospitalizations to begin rising — if they ever do — among the vaccinated before taking action.

The Biden administration has said that booster doses could be rolled out quickly, should the F.D.A. and the Centers for Disease Control and Prevention deem them necessary. An advisory committee to the C.D.C. is scheduled to meet next week to take up the question.

British scientists have recommended giving third doses to adults over 50 and other medically vulnerable people. France, Germany, Denmark and Spain are also considering boosters for older adults or have already begun administering them. Israel is already contemplating fourth doses for its population.

But recent history leaves many experts leery of adding the United States to the list.

Dr. Luciana Borio, a former acting chief scientist at the F.D.A., criticized the Biden administration for announcing a plan for boosters before federal scientists could review the evidence.

The Trump administration pressured scientists at the F.D.A. to authorize hydroxychroloquine and convalescent plasma, for example, without enough evidence to support either treatment. “It seems to me that there’s been a process foul in how we go about making those decisions,” Dr. Borio said.

“We need an F.D.A. that has people making these decisions and retaining that ability to make those decisions independently and based on science alone. If this changes, we all lose.”