WASHINGTON — An expert committee recommended Tuesday that the Food and Drug Administration move to updated coronavirus booster shots targeting some form of the Omicron variant that has dominated for months.

The panel’s vote paves the way for the F.D.A. to push manufacturers to make reformulated boosters in time for the Biden administration to offer them later this year, before an expected winter surge of the virus.

The decision came after a day of hand-wringing about where the virus is headed next and how best to confront it. A top F.D.A. official called it “science at its hardest.” One committee member described the data as “uncomfortably scant.”

And a top official from one of the vaccine manufacturers, Moderna, told the panel that his company would not be able to make the type of doses that the regulators appear to favor until late October or early November, assuming “no data to assess the vaccine at all.”

“None of us has a crystal ball,” Dr. Peter Marks, a top F.D.A. regulator who oversees the agency’s vaccines office, said. “If you do, come over to my house right now. I really would like it.”

Despite all the uncertainties, the panel came down 19-2 in favor of redesigning booster shots to also target Omicron or its subvariants, rather than simply the original version of the virus.

“We’re all troubled by the steady erosion of immune protection,” Dr. Mark Sawyer, an infectious disease specialist with the University of California, San Diego School of Medicine, said. “We’re going to be behind the eight ball if we wait longer.”

The committee debated but did not specify which formulation might work best. The F.D.A. was clearly leaning toward a combination of the existing vaccine and one tailored to two subvariants of Omicron, BA.4 and BA.5. According to new estimates from the Centers for Disease Control and Prevention, those subvariants together now make up more than half of new cases in the country.

That would be a rejection of the booster candidates that Pfizer and Moderna, the two main vaccine developers, have been manufacturing in the expectation that the government would buy doses. The companies had opted for a combination of the existing vaccine and one that targets Omicron itself, not its subvariants.

But Omicron was squeezed out by subvariants almost three months ago, and regulators said in briefing materials that a booster targeting it would be “already somewhat outdated.”

The committee’s recommendation is not binding, but a no vote would have thrown a major wrench in the Biden administration’s efforts to update the existing boosters by the fall. Dr. Marks suggested that the agency would follow with its own recommendation “very rapidly.” With booster contracts worth billions hanging in the balance, the manufacturers are expected to follow the regulators’ wishes.

The meeting highlighted the scientific angst over how to combat a virus that is evolving faster than clinical trials involving humans can deliver results. The panel skipped over the divisive question of who should be eligible for a fall booster.

“Right now the critical thing is the manufacturers need to know what to put into their vaccine,” Dr. Marks said. “Over the coming months, I think we’ll get a sense, and there’ll be plenty of time for debate over who is most appropriate for boosters.”

He predicted that older Americans, at least, might need the next round of protection — a strategy that even some of the more skeptical committee members favor.

Committee members argued for continuing to use the existing vaccines for people who have not yet gotten initial shots. “I don’t think we should lose the prototype,” Dr. Amanda Cohn, a top C.D.C. official, said. “I think it’s a known entity and it’s doing really well in its current job.”

But some panelists questioned whether Moderna and Pfizer, along with its partner BioNTech, had proposed the best booster candidates. Several said that Novavax, a company whose vaccine is not yet even available for initial doses in the United States, presented more impressive data.

Dr. Bruce Gellin, a panel member and the chief of global public health strategy at the Rockefeller Foundation, called for stronger federal leadership. “We need to have better central coordination, not just for those studies, but what the plan is going forward,” he said.

There was little talk about any variant except Omicron, which has proved to be a global game changer.

Jerry Weir, a senior F.D.A. regulator, said that Omicron and its offshoots have dominated for about six months now, and that whatever incarnation of the virus comes next is more likely to be tied to the Omicron family than to earlier versions.

Federal health officials have repeatedly suggested that more advanced vaccines could be needed. As vaccine-induced protection against infection has faded, some Americans have been reinfected even in the span of several months.

In briefing materials, F.D.A. officials said the risk of another major outbreak will rise later this year “due to the combination of waning immunity, further evolution of variants and increased indoor activity.”

Justin T. Lessler, an epidemiologist with the University of North Carolina at Chapel Hill, presented a series of worrisome projections on the U.S. trajectory of the virus. Under the most optimistic scenario, his team anticipated 95,000 deaths in the 12-month period ending March 2023. Under the most pessimistic, the nation would see over 200,000 deaths.

Although the overall death rate is low now compared with earlier in the pandemic, C.D.C. officials said Americans 70 and older have driven increases in hospitalizations. Heather Scobie, an agency epidemiologist, said that despite “rather poor” uptake, second boosters were reducing the risk of death in older adults and that health officials should “be pushing the second boosters in older ages to protect against serious illness.”

While the committee broadly agreed that a fall booster should be reconfigured to take aim at the Omicron family, exactly how was much less clear. Dr. Adam C. Berger, a National Institutes of Health official, voted in favor of an updated booster but said, “I’m not sure that we have evidence to support a change necessarily today.”

Dr. Kanta Subbarao, an adviser to the World Health Organization who presented to the committee, said that she and her colleagues believed a combination of the original vaccine and one targeting Omicron alone could provide a broader range of protection and avert the scramble to keep up with the most current variants. The companies presented data showing that such a formulation increased the level of neutralizing antibodies more than the existing booster, but less than twofold, with only a short follow-up period.

Other experts on the panel said they instead favor targeting the BA.4 and BA.5 subvariants that have become the dominant forms of the virus in the United States. Pfizer presented the only data on a such a formulation.

A company official said preliminary data from a mouse trial showed it worked better against all Omicron’s subvariants than the existing vaccine does. She said researchers do not yet have a side-by-side comparison with a booster targeting Omicron itself.

If regulators choose the subvariant option, it will be especially challenging to deliver doses in time.

Kathrin Jansen, the head of vaccine research for Pfizer, said that regardless of which formulation the F.D.A. chooses, “We are prepared” to deliver doses by early October. But Dr. Stephen Hoge, the president of Moderna, said his company would be able to deliver only by late October or early November — and only “assuming no clinical data requirements.”

Dr. Weir said that although it is difficult to draw broad conclusions from different studies by different companies, overall the data indicated that a booster that incorporates an Omicron component produces a better antibody response against that family of the virus. That offers “the potential for improved vaccine effectiveness,” he said.