WASHINGTON — Johnson & Johnson is planning to ask federal regulators early this week to authorize a booster shot of its coronavirus vaccine, according to officials familiar with the company’s plans. The firm is the last of the three federally authorized vaccine providers to call for extra injections, amid mounting evidence that at least the elderly and other high-risk groups need more protection.

Federal officials have become increasingly worried that the more than 15 million Americans who received the Johnson & Johnson vaccine face too much risk of severe Covid-19. The Food and Drug Administration on Friday scheduled a meeting on Oct. 15 of its expert advisory committee to discuss whether to grant emergency use authorization of a booster shot of Johnson & Johnson’s vaccine.

That is part of a broader effort by the government to shore up the protection provided by all three vaccines. Regulators last month authorized a booster shot for many recipients of Pfizer-BioNTech’s vaccine and are contemplating doing the same this month for recipients of Moderna’s.

The fact that the advisory committee meeting on Johnson & Johnson was scheduled even before the company filed an application to the Food and Drug Administration reflects a particular sense of urgency in the Biden administration to provide more protection to recipients of that vaccine.

Although the federal government has emphasized for months that all three vaccines are highly effective, a recent study by the Centers for Disease Control and Prevention found that Johnson & Johnson’s single-dose vaccine was only 71 percent effective against hospitalization from Covid-19, compared with 88 percent for Pfizer-BioNTech’s vaccine and 93 percent for Moderna’s.

“Real-world data suggest that the two-dose Moderna and Pfizer-BioNTech mRNA vaccine regimens provide more protection” than the single dose of Johnson & Johnson, the researchers said. Other research found that Johnson & Johnson recipients were more likely to have breakthrough infections or symptomatic Covid-19 than recipients of the other two vaccines.

Johnson & Johnson cites some studies with better results: A study of nearly two million people, funded by the company, estimated that the vaccine was 81 percent effective against hospitalization. Other research suggests that protection from Johnson & Johnson’s vaccine does not wane over time like protection from Pfizer-BioNTech’s vaccine.

Nonetheless, Johnson & Johnson now appears to agree with federal officials that a single shot of its vaccine is not enough.

Last month, the company announced that a second dose, given two months after the first, increased the vaccine’s effectiveness against symptomatic Covid-19 by about 22 percentage points, to 94 percent. The firm also said two shots were 100 percent effective against severe disease, although that estimate was less conclusive.

“The data shows that there is a dramatic increase in efficacy when the boost is given at two months. And we believe that it’s potentially even better when given later,” said Dr. Dan H. Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who helped develop the vaccine.

“I’m not a public health authority, but I would say it’s scientifically reasonable for people who are desiring a higher level of efficacy to obtain a booster shot” between two to six months after the first shot of the vaccine, he said.

At a meeting of a C.D.C. advisory committee in September, some independent experts questioned whether Johnson & Johnson recipients were being forgotten while the federal government moved to authorize a third shot for Pfizer recipients.

“To me, the biggest policy question out there is the Johnson & Johnson,” Dr. Helen Keipp Talbot, an internist and infectious disease specialist at Vanderbilt University, said at the meeting last month, where the committee weighed whether to recommend a third shot for some Pfizer recipients. “I worry we’re getting distracted by the question of boosters of Pfizer when we have bigger and more important things to do in the pandemic.”

Dr. Nirav D. Shah, Maine’s top health official and the president of the Association of State and Territorial Health Officials, said that after the Johnson & Johnson vaccine became available this spring, many states directed it to rural areas because it required only one injection and was easier to transport.

“It is rural areas that are getting particularly hit right now” by the Delta variant, Dr. Shah said. “So I think there is good reason to offer J. & J. recipients a booster if the data support it.”

In San Francisco last month, health officials said Johnson & Johnson recipients would be permitted to receive a booster shot of either the Moderna or Pfizer vaccine. “If people talk to their health care provider and say, ‘I’d really like to have an mRNA shot in addition to the Johnson & Johnson shot that I got,’ we will accommodate that,” Dr. Grant Colfax, the city’s public health director, told reporters.

For federal regulators and the government’s panels of outside experts, key questions remain: How long after an initial Johnson & Johnson injection should a second shot be administered? And would it be better to boost Johnson & Johnson recipients with a shot of Moderna’s or Pfizer’s vaccine?

If the Food and Drug Administration authorizes a second shot six months after the initial injection, that would allow the company to market its vaccine as a one-shot regimen, plus a booster. Some data indicate a second shot at six months produced a stronger immune response than a second shot at two months.

Researchers found a nine- to twelvefold increase in antibodies among clinical trial recipients who received a second shot after six months, compared with a roughly fourfold increase for people who received a second dose after two months, Dr. Barouch said.

But there could be a trade-off: Some argue that Johnson & Johnson recipients would not be sufficiently protected during the extra months before a second shot, and that a two-dose regimen made more sense from the start.

The timing is not a trivial question: According to C.D.C. data, more than two-thirds of Johnson & Johnson recipients were vaccinated at least four months ago. Many would not be immediately eligible if a booster shot was available only six months after the first injection.