Federal health officials are racing to ensure that millions of Americans with weakened immune systems can get additional shots of coronavirus vaccines to protect them against the highly contagious delta variant.

The actions could mean the extra shots would be authorized in days or weeks, according to federal officials who spoke on the condition of anonymity because the plan has not been announced.

The stepped-up activity reflects increased urgency by the Biden administration to shield some of the nation’s most vulnerable adults as coronavirus cases rise sharply. That has increased as some other countries take steps to provide shots to people who are immunocompromised or older and as some Americans pursue the shots on their own.

In the next week or two, the Food and Drug Administration is expected to review data from the Centers for Disease Control and Prevention supporting the use of additional vaccine doses for the immunocompromised. If officials are persuaded, they will amend the emergency use authorizations for the vaccines to permit the extra inoculations. Advisers to the CDC and the agency, in turn, will urge people with certain medical conditions to talk to their doctors or pharmacists about getting the shots.

Immunocompromised patients represent about 7 million adults in the United States, including those who have received organ transplants, patients on cancer treatments and those with rheumatologic conditions and HIV, according to the CDC. They are more likely to become seriously ill from COVID-19, the disease caused by the coronavirus, and might more frequently spread the virus to others, experts say.

“It is extremely important for us to move to get those individuals their boosters and we are now working on that,” Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, said Thursday at a White House briefing. He said that many immunosuppressed people did not have a response to the vaccine “that we feel would be adequately protective.”

The action on the immunocompromised population comes as officials scramble to develop a more comprehensive strategy for vaccine boosters for others, including older people. But those plans won’t be completed until September, when additional data is collected, and would be put into effect only if officials conclude that boosters are needed more broadly because of waning immunity. Those shots would probably not be rolled out until October, they said.

Both the Pfizer-BioNTech and Moderna vaccines are now authorized for a two-dose regimen and Johnson & Johnson for one shot. The goal is to permit extra doses of all three vaccines, but officials said that decision would depend on the data. They also are unsure what the data will show about mixing and matching — using a Pfizer or Moderna vaccine, for example, to bolster Johnson & Johnson’s shot.

Israel is already offering a third dose of the Pfizer-BioNTech coronavirus vaccine to severely immunocompromised adults. Germany said it also would start offering boosters to the elderly, the immunocompromised and those who got the AstraZeneca or Johnson & Johnson shots, because studies suggest they may be less effective than other vaccines.

France also plans to make certain groups of people — residents of nursing homes, those over age 75 and people with severe health conditions — eligible for additional doses in September. Britain has announced a plan for additional doses to begin in September, targeted first to people who are immunocompromised, but is awaiting guidance from an expert panel before firming up plans.

In small studies, reactions to a third dose of mRNA vaccines were similar to prior doses, according to CDC.

Last month, Camille Kotton, a transplant-medicine physician at Massachusetts General Hospital and a member of the CDC’s Advisory Committee on Immunization Practices, said many immunocompromised patients have “taken matters into their own hands and many are proceeding with additional doses of vaccine as they see fit,” which she called “really challenging.” She said at a meeting of the advisory committee that she was concerned about patients getting extra shots in an unsupervised fashion.

The revised clearances would give doctors and providers liability protection for administering the extra shots, officials said. Authorizing an extra dose of the Pfizer-BioNTech vaccine for the immunocompromised is less critical because that product is likely to get full FDA approval in the next two to four weeks. Once that happens, doctors are permitted to use the vaccine as they see fit, as long as it is reasonable.

Even though Fauci and others sometimes use the word “booster” in discussing an extra shot for the immunocompromised, other health officials argue that technically they are not boosters. Instead, they say, they are part of the original primary series of vaccines for people who have failed to respond fully or at all to the shots.

A booster in effect “reinvigorates the immune response by giving it another stimulus,” said William Schaffner, an infectious-disease physician at Vanderbilt University and a liaison member to the CDC advisory panel. “Whereas in an immunocompromised person, they never achieved that appropriate immune response, so people are beginning to think of it as a three-dose regimen for immunocompromised people.”

The CDC vaccine advisory panel is scheduled to meet next Friday to hear presentations on additional data about doses for the immunocompromised in anticipation of changes in FDA regulatory authorization, according to a federal health official. That way, once the FDA takes action, the advisory panel could make formal recommendations for use.

At last month’s advisory panel meeting, CDC vaccine expert Sara Oliver said evidence is needed regarding the safety and strength of a mixed-dose approach in immunocompromised people. She noted that studies from Europe about mixed doses, including a large United Kingdom study in the general adult population, found that the response to one dose of AstraZeneca and one dose of Pfizer-BioNTech to be equivalent to two doses of AstraZeneca.

Members of CDC vaccine advisory panel have spoken of the urgent need to give people with fragile immune systems additional doses amid growing concerns about waning immunity in vulnerable populations and surging delta infections.

At one point, the CDC was exploring making additional doses possible through an FDA program that gives people access to drugs that have not yet been approved. But officials concluded it would be more straightforward to amend the emergency use authorizations.

Determining eligibility is a major issue to be addressed. “How do you define immunocompromised, and how do you make that clear to the public and the medical professions and how do you implement it in practice,” Schaffner said.

Some people who have sought and gotten extra shots already say that they have not been asked whether they are already vaccinated.

“Basically we have an honor system,” an official said. “But that’s OK, this isn’t fentanyl.”

related stories

• US military concludes working week with 43 coronavirus cases in Japan, South Korea
• US military on Guam reimposes mask mandate as island’s coronavirus cases spike