WASHINGTON — A key federal advisory committee voted unanimously Friday to recommend Johnson & Johnson booster shots, most likely clearing the way for all 15 million people who got the company’s one-dose coronavirus vaccine to receive a second shot.

If the Food and Drug Administration and the Centers for Disease Control and Prevention accept the recommendation, as expected, boosters could be offered by late next week. But many committee members made clear that they believed Johnson & Johnson recipients might benefit from the option of a booster of the Pfizer-BioNTech or Moderna vaccine, something a top F.D.A. official said the agency was considering.

With a series of votes over the past month to recommend boosters for all three coronavirus vaccines used in the United States, the panel set aside significant divisions and skepticism about whether extra shots are needed and edged ever closer to the goal that President Biden laid out in August when he called for boosters for all adults.

Well over 100 million fully vaccinated people will be eligible for boosters if the F.D.A. and C.D.C. endorse the committee’s latest recommendations, even though some scientists say that the evidence supporting boosters remains weak and that it would have been wiser to focus on reaching the unvaccinated, including abroad.

Johnson & Johnson’s vaccine took a beating at Friday’s session, as did the F.D.A. for pushing for a decision without verifying all of the data that the company had submitted.

But the panel members appeared swayed by the argument that it would be unfair to deny Johnson & Johnson recipients an additional shot after endorsing boosters for recipients of the other two vaccines, especially in the face of evidence that Johnson & Johnson offers the weakest protection of the three.

“There is a public health imperative here, because what we’re seeing is that this is a group with overall lower efficacy than we have seen with the mRNA vaccines,” said Dr. Arnold Monto, the committee’s acting chairman and a professor of epidemiology at the University of Michigan School of Public Health. “So there is some urgency there to do something.”

In its second successive day of discussion on boosters, the group wavered on whether it would be wise to soon offer extra shots to younger recipients of the Pfizer and Moderna vaccines.

The government’s strategy now is to offer boosters of those vaccines to people 65 and older and to younger adults at high risk because of their medical conditions or jobs. No decision was made about broader eligibility criteria, but Dr. Monto and others described the issue as increasingly pressing.

The panel also seemed intrigued by preliminary data suggesting that Johnson & Johnson recipients may be better off with a booster shot from Moderna or Pfizer. Although no vote was taken, Dr. Peter Marks, who oversees the F.D.A.’s vaccine division, said regulatory action to allow boosters with a different vaccine was “possible.”

While some experts emphasized that the data was based on small groups of volunteers and short-term findings, others urged the F.D.A. to move quickly with what has fast become known as a mix-and-match approach, especially for recipients of Johnson & Johnson’s vaccine, which is much less widely available.

“I’m sold already,” said Dr. Mark Sawyer, an infectious disease specialist with the University of California San Diego School of Medicine. “We need flexibility and to improve access to everyone.”

Others said they worried that the public would end up bewildered if the government kept broadening the categories of people eligible for boosters and which vaccine could be used for extra shots.

“I hope we can do this in a way that doesn’t look like we’re changing rules all the time,” said Dr. Stanley Perlman, a professor of immunology at the University of Iowa.

Health officials and committee members suggested on Friday that the single-shot Johnson & Johnson vaccine had long been less protective. In a particularly biting critique, Dr. Amanda Cohn, a high-ranking C.D.C. medical officer, said a single dose of Johnson & Johnson’s vaccine offered less protection than two doses of the mRNA vaccines made by Pfizer or Moderna — a gap that would only grow if it remained a one-shot regimen while the other two-shot vaccines were followed by a booster.

Dr. Marks emphasized that the one-shot, easily stored Johnson & Johnson shot had served partly as an “outreach” vaccine — suggesting that refusing to offer a booster would disadvantage vulnerable populations.

To date, more than 104 million people in the United States have been fully vaccinated with Pfizer’s product, more than 69 million with Moderna’s and about 15 million with the Johnson & Johnson shot.

The government decided last month to offer booster shots of Pfizer’s vaccine to older Americans and other high-risk groups, and the panel voted Thursday to recommend the same approach for Moderna recipients. More than eight million people in the United States have already obtained Pfizer booster doses, and about 1.6 million have received third doses of Moderna’s, even though only Moderna recipients with immune deficiencies are officially eligible.

For Johnson & Johnson recipients, the committee recommended an additional shot for everyone who had received the vaccine — a reflection of its lower efficacy. Although the added injection was cast as a booster, some experts on the committee argued that the vaccine should have involved two doses from the start, separated by about two months.

Johnson & Johnson officials said a second dose given either two or six months after the first shot increased antibody levels, part of the immune response to vaccines. They also said that unlike Pfizer’s vaccine, Johnson & Johnson’s potency did not wane significantly over time.

The experts generally agreed that the protection conferred by a single dose was inadequate, but at least some were unconvinced that the second dose would bolster that protection significantly.

They were critical of the company’s data, saying F.D.A. regulators had challenged the sensitivity of one key test used to measure antibodies in people who received booster doses. They balked at the size of a study on Johnson & Johnson booster doses given at a sixth-month interval, based on only 17 volunteers. And they were clearly distressed to see slide after slide of data with the huge caveat: “Not verified by F.D.A.”

Dr. Archana Chatterjee, the dean of the Chicago Medical School, pointedly asked why the F.D.A. was pushing for a decision without a thorough review.

But she said later that she was convinced by the totality of the data, especially a C.D.C. report that found Johnson & Johnson was only 71 percent effective against hospitalization, compared with 88 percent for Pfizer and 93 percent for Moderna.

Ahead lie still more decisions for the F.D.A. and its outside experts. The group is set to meet again this month to discuss Pfizer-BioNTech’s request for authorization of a lower dose of its vaccine in children 5 to 11. Also pending is a decision on whether to authorize Moderna’s vaccine for adolescents — a move complicated by conflicting data about possible side effects involving the heart.

Regulators are expected to soon consider whether to allow people to get boosters of a different vaccine than what they initially got, which might produce a stronger immune response. “It’s real-world, practical questions that people want to know,” said Dr. Kirsten Lyke of the University of Maryland School of Medicine, who presented the results of the mix-and-match study to the committee.

Preliminary findings showed Johnson & Johnson recipients who got a booster with the Moderna vaccine saw their antibody levels rise 76-fold within 15 days, compared with only a fourfold rise if they received a booster shot of Johnson & Johnson. A Pfizer-BioNTech booster raised antibody levels in Johnson & Johnson recipients 35-fold.

The trial only looked at antibody levels, which on their own are an insufficient measure of how well different combinations of vaccines would protect people.

Dr. Cohn, the C.D.C. official, suggested that the F.D.A. could include language in its booster authorizations that would allow switches between brands.

“From a public health perspective, there’s a clear need in some situations for individuals to receive a different vaccine,” she said. She said that some people might not have access to the same vaccine again, or might have increased risks of some side effects from using it again as a booster.

Dr. Ofer Levy, an infectious disease physician at Boston Children’s Hospital, said the F.D.A. should act because some people were already seeking boosters on their own.

“In the real world, all these kinds of combinations or extra boosters are already happening,” he said. “We can’t hide from it, and I do think we need to give guidance to the public.”

At the same time, the government needs “to make sure that we don’t confuse the public even more than we are already,” said Dr. Jeannette Lee, a professor at the University of Arkansas for Medical Sciences.