An influential scientific panel on Thursday opened a new front in the campaign against the coronavirus, recommending booster shots of the Pfizer-BioNTech Covid vaccine for a wide range of Americans, including tens of millions of older people. But the experts declined to endorse additional doses for health care workers, teachers and others who might have higher exposure on the job.

The decisions were made by the C.D.C. panel, the Advisory Committee on Immunization Practices, in a series of votes, during which scientists agonized over their choices. The recommendations revealed deep divisions among federal regulators and outside advisers about how to contain the virus nearly two years into the pandemic.

Just a day earlier, the Food and Drug Administration authorized booster shots for certain frontline workers. But the C.D.C.’s advisers disagreed that the doses were needed by so many healthy people.

The next step is for Dr. Rochelle Walensky, the director of the C.D.C., to make a formal recommendation. If she follows the guidance of the agency’s advisory committee, as is typically the case, the agency’s guidance may conflict with that of the F.D.A.

One administration official said Xavier Becerra, the secretary of health and human services, might ultimately have to mediate between the two agencies.

“There’s a complexity here, because Dr. Walensky was part of the White House announcement” on boosters, said Dr. Ashish Jha, dean of the Brown University School of Global Health. “I think she’s going to feel some amount of pressure to authorize this for health care workers.”

Depending on what is decided, the White House is likely to begin promoting and rolling out a plan for booster shots as soon as Friday. That would be in keeping with the administration’s previously announced plan to offer the additional doses the week of Sept. 20.

Whatever the scientific reservations, millions are expected to seek out booster shots. In one recent poll, about three-quarters of vaccinated Americans said they would opt for a booster if the doses were available.

State health departments generally follow the recommendations of the C.D.C. But many Americans were scrambling for boosters even before the F.D.A.’s authorization, typically by finding a cooperative pharmacist or by claiming to be unvaccinated.

The C.D.C.’s advisers acted on what they described — with considerable frustration — as scant research, mulling over conflicting data points that seldom pointed in one direction.

In the end, the panel unanimously endorsed booster shots for adults over 65 and for residents of long-term care facilities, who most clearly will benefit.

The committee also backed the shots for people 50 to 64 with medical conditions that leave them at risk for severe Covid-19, as well as those 18 to 49 who have certain medical conditions, based on an assessment of their individual needs.

Only Americans who already have received two doses of the Pfizer-BioNTech vaccine will qualify for booster shots. The panel was not asked to judge whether people who received the Moderna and Johnson & Johnson vaccines should receive the additional doses, which have not been authorized by the F.D.A.

Several experts on the C.D.C. panel nevertheless urged a mix-and-match strategy, saying that they could see little reason not to offer a Pfizer-BioNTech booster to someone who qualified but had received, for example, the J. & J. vaccine. Some members warned that delivering multiple rounds of booster shots, available periodically when authorized, would tax an already burdened health care system.

The C.D.C. panel’s guidance followed weeks of internal disagreement and public debate among American health officials and advisers. In mid-August, President Biden announced plans for a booster rollout, but scientists and regulators were quick to point out there was little research on who might benefit and how the doses should be distributed.

The F.D.A.’s acting commissioner, Janet Woodcock, said on Wednesday that the agency’s authorization would allow for booster doses “in certain populations such as health care workers, teachers and day care staff, grocery workers and those in homeless shelters or prisons, among others.”

But some members of the committee said there was little evidence to suggest that vaccinated teachers, and even health care workers, were at risk of repeated exposure to the virus. The decision reflected fears that such a broad recommendation would effectively throw the doors open to an all-adults booster campaign.

“My sense was that the committee felt that that was sort of a hole that you could drive a truck through,” Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the F.D.A.’s vaccine advisory panel, told reporters at an online briefing on Thursday.

Over the two days, the panel wrestled with the public’s expectations for Covid vaccines, the safety of third doses and how a booster program would affect nursing home residents. Booster doses alone would not turn back the pandemic, some scientists noted: Only vaccinating the unvaccinated would do that.

“We may move the needle a little bit by giving a booster dose to people,” said Dr. Helen Talbot, an associate professor of medicine at Vanderbilt University. But, she added, “the hospitals are full because people are not vaccinated.”

The advisers also grappled with a lack of clarity on the goal of the vaccines: Should it be to prevent all infections, or to forestall severe illness and hospitalization?

Many suggested that booster doses could do only the latter, and that trying to thwart all infections was impossible. That reasoning supported limiting who should receive the doses, the experts said.

On Thursday, C.D.C. scientists presented models indicating that, if booster doses were to slightly increase people’s protection against hospitalization, the additional shots could prevent more than 2,000 hospitalizations for every million doses given.

But it was not clear how long additional protection from a booster would last, raising the prospect that boosters would need to be given repeatedly.

Boosters can reduce infections in nursing home residents, who are among those at highest risk. Even so, cases in nursing homes will persist when community transmission is high, according to a modeling study presented at the meeting.

The advisers also wrestled with the practicalities of endorsing a booster shot for only Pfizer-BioNTech recipients, when close to half of vaccinated Americans have received Moderna or J. & J. vaccines.

“I just don’t understand how, later this afternoon, we can say to people 65 and older, ‘You’re at risk for severe disease and death, but only half of you can protect yourselves right now,’” said Dr. Sarah Long, a pediatrician and infectious diseases expert at Drexel University College of Medicine in Pennsylvania.

Committee members also expressed concern on Thursday that some recommendations — particularly that certain younger Americans be allowed booster shots after an assessment of individual risks — would mean that only the wealthy and educated would gain access to additional shots.

Some experts seemed to suggest on Wednesday that it might be better to hold off on recommending any booster shots until recipients of all three vaccines could qualify for them.

Moderna’s booster authorization may arrive in a few days to weeks. The company has applied to the F.D.A. for authorization of a booster shot carrying half the dosage given in the first two shots, which has complicated the agency’s deliberations.

Some global health experts have criticized the Biden administration for pushing booster shots when much of the world has yet to receive a first dose. But analysts noted that even if the United States distributes booster shots, there should still be considerable excess vaccine supply this year, and they urged the government to begin sending the extra doses abroad.

Sheryl Stolberg contributed reporting from Washington.