WASHINGTON — The Biden administration is moving ahead with a plan to give at least everyone 65 and older — and possibly some younger adults as well — the option of a second booster of the Pfizer-BioNTech or Moderna coronavirus vaccine without recommending outright that they get one, according to several people familiar with the planning.

Major uncertainties have complicated the decision, including how long the protection from a second booster would last, how to explain the plan to the public and even whether the overall goal is to shield whoever is deemed eligible only from severe disease or from less serious infections as well, since they could lead to long Covid.

Much depends on when the next wave of Covid infections will hit, and how hard. Should the nation be hit by a virulent surge in the next few months, offering a second booster now for older Americans could arguably save thousands of lives and prevent tens of thousands of hospitalizations.

But if no major wave hits until the fall, extra shots now could turn out to be a questionable intervention that wastes vaccine doses and that could deepen vaccination fatigue and sow doubt about the government’s strategy. The highly contagious Omicron subvariant BA.2 is helping to drive another surge of coronavirus cases in Europe and is responsible for about a third of new cases in the United States, but health officials have said they do not anticipate a major surge caused by the subvariant.

Federal health officials have hotly debated the way forward, with some strongly in favor of a second booster now and others skeptical. But they appear to be coalescing around a plan to give at least older Americans the option, in case infections surge again before the fall. It was unclear how broad the group would be. In the fall, officials say, Americans of all ages should get another shot.

A decision from the Food and Drug Administration on whether to authorize a second booster could come early next week, according to multiple people familiar with the deliberations. The Centers for Disease Control and Prevention might then advise those eligible to consider getting another shot, rather than recommending they do so.

Giving some people the option of a second booster would be at best a stopgap measure. Many experts argue that the existing coronavirus vaccines need to be modified because the virus’s variants are diminishing their power; the question is how to reconfigure them. A surge in the fall is considered highly likely, whether it comes in the form of the Omicron variant, a subvariant like BA.2 or a new lineage entirely.

More than a dozen studies are underway to find the next generation of vaccines, with the first results expected in May or June. If all goes well, that would allow enough time to produce new doses before the fall. One major hitch is that the Biden administration says it does not have the money it needs to reserve its place in line by paying vaccine manufacturers for doses in advance.

On the plus side, data from the C.D.C. indicates that four to five months after a third shot, the Pfizer-BioNTech and Moderna vaccines remain about 78 percent effective against hospitalization due to Covid-19. That might even be an underestimate, because the study may have included a disproportionate number of people with immune deficiencies that make them more likely to become severely ill.

On the other hand, 78 percent is still a drop from the 91 percent effectiveness that was found after two months, and the vaccines’ potency may fall further with more time. If a new wave hits in May or over the summer, even a somewhat modest decline in protection against hospitalization could have a huge impact on the nation’s roughly 55 million Americans 65 or older, who have borne the brunt of the pandemic. Pfizer and BioNTech have said emerging data, including from Kaiser Permanente, shows that the potency of its booster dose against severe disease wanes in three to six months.

Already, one in 75 Americans 65 or older has died of Covid, making up three-fourths of the nation’s deaths from the virus, according to the C.D.C.’s data. More than 33 million people in that age group, or more than two-thirds, have received a first booster and would be eligible for a second.

For some officials, the bottom-line question is this: How much should effectiveness against hospitalization drop before a second booster is justified for those most at risk?

As it was in the fall, when boosters were first rolled out, the broader scientific community is divided over the policy. “I am not persuaded there is substantial waning of protection against severe disease after the third dose,” Dr. Philip Krause, a former senior regulator at the F.D.A., said in an interview.

Dr. Monica Gandhi, an infectious disease doctor and medical professor at the University of California, San Francisco, said that healthy younger people with one booster were fine at this point, but that older people “should probably start receiving fourth shots now.”

There may be somewhat less resistance among scientists now than there was to the first booster shots, since evidence has emerged that those doses saved lives during the winter’s Omicron wave.

Given the limited nature of data supporting a second round of booster shots, some federal officials say that some sort of neutral advice is as far as the Biden administration can go. But in general, wishy-washy regulatory advice is not popular, as people and doctors often want concrete advice more than options.

Dr. Judith A. Aberg, chief of the division of infectious diseases at Mount Sinai Health System, said the public could be frustrated with mere permission for a second booster.

Unlike with the first round of regulatory decisions on booster shots, no meetings of the advisory committees of either the F.D.A. or the C.D.C. are planned ahead of the decision on second boosters. The panels’ recommendations are nonbinding but are usually followed. Bypassing those committees will draw criticism.

“This is a complex decision that involves a pretty deep dive, and I think it would really benefit from public discussion,” said Dr. Jesse L. Goodman, a former chief scientist at the F.D.A. “I would not want to see an advisory committee skipped on this.”

But administration officials seem willing to accept complaints about the process. The F.D.A. has scheduled an April 6 meeting of its advisory committee to discuss what the administration’s overall vaccine strategy should be moving forward.

As for timing, federal officials appear to be simply making their best guess. If people get a second booster now and the virus has a resurgence in July, their protection may have already fallen off again. On the other hand, if the administration waits until a Covid wave hits, it will be too late to vaccinate tens of millions of people.

The supply is there, at least for older people: States have 131 million doses of the Pfizer-BioNTech and Moderna vaccines on hand.

Many experts say there is no evidence that an additional shot could hurt people’s immunity by habituating them to coronavirus vaccines.

The biggest downside may be more vaccine fatigue and skepticism that the vaccines work and that the nation’s vaccine policy is really driven by data. With each successive shot that becomes available, fewer Americans get it. On the other hand, authorizing a second booster might prompt more people to get the first one, Dr. Aberg of Mount Sinai said.

Besides the C.D.C., Britain and Israel have put out data on the waning effectiveness of booster shots. The latest report by Britain’s health security agency states that effectiveness against symptomatic infection drops to between 25 and 40 percent 15 weeks or longer after a booster dose of either Pfizer or Moderna.

But the British health agency said how well boosters protect against hospitalization was harder to measure. Since Omicron typically caused milder illness than previous variants, more hospitalized patients tested positive for Covid but had been admitted for other reasons.

Looking only at patients admitted for respiratory illness, the agency estimated that vaccine effectiveness against hospitalization for those 65 or older dropped to 85 percent from 91 percent 15 weeks or more after the booster. Like a smattering of other countries, Britain is offering a second booster to older people and others at high risk this spring.

Israel’s data suggests that a second booster shot raises protection fourfold against hospitalization and twofold against infection. But no one knows for how long. Since Israel only recently began its second booster campaign, it has data for only two months or less. Other Israeli data suggests that a second booster restores antibody levels to their peak level after the first booster, but Dr. Aberg said that data set, too, had limitations.

Neither Pfizer nor Moderna seem to have much of their own data to support their requests for emergency authorization; Pfizer is seeking second boosters for those 65 and older, while Moderna filed a sweeping request to offer second booster shots to all adults. Neither has submitted data from a randomized, placebo-controlled study — considered to be the gold standard of scientific evidence — on how well the dose would work.

“We are going to have to make this decision on the basis of incomplete information,” said Dr. Peter J. Hotez, a vaccine expert with the Baylor College of Medicine in Houston.

Sheelagh McNeill and Kitty Bennett contributed research.