WASHINGTON — The Food and Drug Administration authorized booster shots on Wednesday for tens of millions of recipients of Moderna’s two-dose coronavirus vaccine and Johnson & Johnson’s single-dose shot, significantly expanding efforts to bolster protection for vulnerable Americans.

The agency also authorized medical providers to give people a booster shot of a different Covid-19 vaccine, a strategy known as “mix and match.”

That decision may dampen interest in the United States in Johnson & Johnson’s vaccine, which studies have found provides less protection than the other two. Recipients of that vaccine will have the option of seeking a Moderna or Pfizer-BioNTech booster, which could prompt a more substantial jump in protective antibodies. Recipients of the other two vaccines will have the same leeway to choose a different vaccine for a booster shot.

The regulators, who authorized boosters for high-risk recipients of the Pfizer-BioNTech vaccine last month, did not recommend any one vaccine over another as a booster.

“We do not have preferential recommendations,” Dr. Janet Woodcock, the F.D.A.’s acting commissioner, said at an evening news briefing. “We feel that if patients have questions, they should consult with perhaps their physician or another provider.”

The latest authorizations arrived amid what is shaping up to be a busy stretch of major regulatory decisions on vaccines. The F.D.A. is expected to decide in the coming weeks whether Pfizer-BioNTech’s vaccine should be authorized for children 5 to 11. The agency’s independent vaccine advisory committee is set to consider the matter on Tuesday and make a recommendation.

Regulators may also decide as early as November whether to allow even more people to obtain booster shots, including younger adult recipients of the Pfizer or Moderna vaccines who are not yet eligible. While many vaccine experts have questioned whether healthy younger people need booster shots at this time, some members of the advisory committee urged the agency last week to expand eligibility.

Dr. Peter Marks, the F.D.A.’s top vaccine regulator, said the agency could be “nimble” in including more younger adults. “It is something that can happen as soon as we see and feel that we need to take that action,” he said.

One key question before regulators was whether to authorize a full or a half-dose of Moderna’s vaccine as a booster shot. Data from a federally funded study suggested a full-strength dose of Moderna could hugely boost the antibody levels of Johnson & Johnson recipients and increase the antibody levels of Pfizer recipients better than an additional shot of the same vaccine.

But Moderna only sought permission for a half-dose as a booster shot, submitting data in support of that. In the end, the F.D.A. decided to authorize the half-dose Moderna booster for recipients of all three vaccines, at least in part, Dr. Marks suggested, in order to limit confusion among providers.

The move on Wednesday to make a much larger swath of Americans eligible for boosters followed unanimous votes last week from the F.D.A.’s advisory committee to recommend the authorizations. A separate committee advising the Centers for Disease Control and Prevention is scheduled to vote on Thursday on its own recommendations for the Moderna and Johnson & Johnson boosters.

The panel will also consider whether people need more guidance about when and whether to switch vaccines for a booster shot. C.D.C. officials are expected to argue for flexibility.

Given the existing data, “we have to be noncommittal about what is the best,” Dr. Marks said. “We have to simply say that any one of these combinations is reasonable.”

The C.D.C.’s director typically endorses that panel’s recommendations as a final step before vaccines are put into use. If the agency agrees with the F.D.A., those booster shots could be available to the public as soon as this weekend.

The eligibility requirements for a booster differ depending on vaccine. Pfizer and Moderna recipients will be able to get an extra shot six months after their second injection if they are at least 65 years old or at high risk of severe Covid-19 complications because of medical conditions or where they work. Johnson & Johnson recipients — all of whom are 18 and older — will be eligible for a second shot at least two months after the first.

The decision to allow recipients to switch to a different vaccine as a booster adds yet another layer of complexity. “Although it is not simple,” Dr. Marks said, “it’s not utterly hopelessly complex.”

Even as the F.D.A.’s advisory panel last week unanimously supported Moderna and Johnson & Johnson boosters last week, its experts raised concerns that the data in the companies’ applications was limited and, in the case of Johnson & Johnson’s, not independently verified.

Moderna argued to the committee that a third shot of its vaccine would bolster protection against infection and more moderate disease. There appeared to be little evidence that the vaccine’s robust protection had waned significantly.

In contrast, some F.D.A. experts and committee members argued that Johnson & Johnson recipients needed an additional shot to better bolster them against severe Covid-19, since that vaccine was less effective than those of Moderna and Pfizer-BioNTech.

The F.D.A. discussed data with the committee showing that Johnson & Johnson’s vaccine was only roughly 70 percent effective against hospitalization, compared with around 90 percent for Moderna and Pfizer.

But other data, including from a study of nearly nine million people in New York State, found better results from a single dose of Johnson & Johnson, including for older Americans, with durable protection.

The experts appeared swayed by the idea that a single shot of Johnson & Johnson’s vaccine never offered as much protection as the vaccines made by Moderna and Pfizer-BioNTech, and thus needed to be shored up. They also argued that Moderna recipients should be eligible for booster shots, at least partly because Pfizer recipients already were. Pfizer won authorization for a booster shot last month, after it argued that the potency of its vaccine had waned against severe disease.

“We’ve already approved it for Pfizer, and I don’t see how we can possibly not approve it for Moderna” without sowing confusion, said Dr. Stanley Perlman, an infectious disease expert at the University of Iowa and an F.D.A. committee member.

Norman W. Baylor, the former director of the F.D.A.’s vaccines office, said that the lack of substantiated data for Johnson & Johnson’s vaccine was highly unusual, calling into question why the F.D.A. had pushed to gather its expert committee for a vote on booster shots.

“It’s obvious F.D.A. was not as prepared,” he said. “The F.D.A. appears to be, from what we’ve seen, rushed to make a decision on these boosters.”

After the Biden administration began pushing for a broad booster rollout in August, top officials said that streamlining booster recommendations could help avert public confusion. Dr. Marks has said the goal was to reach a “harmonized” approach toward booster recommendations for the general population.

For at least some of the roughly 15 million people who chose the Johnson & Johnson vaccine, the debate over the company’s data may not matter. The F.D.A.’s decision to allow flexibility to providers and patients in booster shots could lead many of them to choose a Moderna or Pfizer-BioNTech booster instead.

Regulators were influenced by preliminary data from a federally funded study that measured antibody levels in people who received a different vaccine as a booster, comparing nine groups of 50 volunteers each. Those getting another dose of Johnson & Johnson saw antibodies go up just fourfold, the study found. Switching to a Pfizer-BioNTech booster raised antibody levels by a factor of 35. A full-dose Moderna booster raised them 76-fold.

Dr. Kirsten E. Lyke of the University of Maryland School of Medicine, who presented the findings at the F.D.A. meeting last week, cautioned in an interview against drawing hasty conclusions from their results. By next month, the researchers hope to know how well the different boosters increase virus-attacking T cells. It is possible that Johnson & Johnson’s vaccine could do well in those results, she said, adding, “We’ll get a more rounded picture.”

Since the Pfizer-BioNTech vaccine was authorized as a booster shot last month, some state health officials have clamored for mix-and-match flexibility, saying it would help them reach older and at-risk residents in cases where they did not have a supply of the same vaccine that people initially received.

Top C.D.C. officials suggested last week that the mix-and-match rules would also leave room for those concerned about side effects — for example, younger women worried about the Johnson & Johnson vaccine’s ties to a rare blood clotting condition — to switch brands. Some may be interested in switching to Johnson & Johnson, on the other hand, if they have concerns about rare heart-related side effects linked to the Moderna and Pfizer-BioNTech vaccines.

“We would expect many people will continue to get the same series that they had already received,” Dr. Woodcock said. But she added: “We wanted to provide a lot of flexibility.”

Carl Zimmer contributed reporting from New Haven, Conn.