Regulators took steps Thursday to shore up the defenses of millions of vulnerable Americans against the coronavirus, authorizing extra doses of two widely used vaccines for some people with weakened immune systems.

The action by the Food and Drug Administration means that additional shots could be available as soon as this weekend for patients who have received organ transplants or have certain types of cancer or other illnesses.

The move was applauded by medical experts worried about people unable to generate robust anti-virus responses even after being fully vaccinated. Details about how the shots will be administered — and who exactly will be eligible for them — are expected to be hashed out Friday during a meeting of Centers for Disease Control and Prevention advisers.

The FDA authorization came as Biden administration officials appear increasingly convinced that boosters for the broader population will be necessary. Officials are scrutinizing data on the vaccines’ durability and discussing when extra shots might need to be administered and which groups might get priority, according to federal officials who spoke on the condition of anonymity because they were describing internal discussions. Those officials said the discussions have become increasingly urgent as the delta variant wreaks havoc in many parts of the country and some studies indicate a waning of the vaccines’ protection over time.

“No vaccine, at least not within this category, is going to have an indefinite amount of protection,” Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said Thursday on NBC’s “Today” show. “Inevitably, there will be a time when we’ll have to get boosts.”

The FDA decision updated the emergency use authorizations for the Pfizer-BioNTech and Moderna coronavirus vaccines to say an additional dose could be given to some immunocompromised individuals, specifically organ transplant recipients and others with similar levels of immune-system impairment. The agency did not give further details. The Johnson & Johnson vaccine was not included because there was not sufficient data on the issue, according to the agency.

About 2.7 percent of U.S. adults, according to the CDC, or about 7 million people, are immunocompromised. Some, like transplant patients, take immune-suppressing drugs to prevent organ rejection. Others have blood cancers and other illnesses that damage the immune system. Still others are on cancer chemotherapy. Making such people eligible for an extra shot, doctors say, is preferable to having worried patients seek out additional inoculations on their own. Not all people with impaired immune systems will need an additional dose as of now, experts say.

The CDC’s Advisory Committee on Immunization Practices is expected to vote to recommend the extra doses on Friday and to give further details on how they will be administered. The panel also is likely to urge patients to talk to their doctors about the additional shots. CDC director Rochelle Walensky is expected to sign off on the decision Friday afternoon, according to federal health officials who spoke on the condition of anonymity because they were not authorized to speak publicly.

In addition, the panel is scheduled to discuss what kind of evidence would be needed to decide whether boosters are needed for the broader population, and which groups — such as older Americans — should get them first, according to a federal health official. Israel has already started administering booster vaccines to older people, and other countries are laying plans to do so.

The FDA action “is the right thing to do,” said Mounzer E. Agha, a hematologist at the University of Pittsburgh Medical Center who treats patients with multiple myeloma, lymphoma and other blood cancers.

Immunocompromised people are more likely to become seriously ill from covid-19, the illness caused by the coronavirus; to transmit the virus to household contacts; and to have breakthrough infections. These people also can potentially spawn mutations that lead to dangerous new variants, experts say.

Studies have shown for months that many immunocompromised people have tepid or no responses to the vaccines, even after a full regimen.

“The country has entered yet another wave of the COVID-19 pandemic, and the FDA is especially cognizant that immunocompromised people are particularly at risk for severe disease,” acting FDA commissioner Janet Woodcock said in a statement Thursday. “After a thorough review of the available data, the FDA determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna vaccines.”

The advisory panel and the CDC are not going to require patients to have a prescription, according to federal officials who spoke on the condition of anonymity because the plan has not yet been announced. Consumers will simply have to attest that they have impaired immune systems. The goal is to minimize barriers to vaccination, the officials said. The additional doses will be free.

But some experts have said there are sure to be questions about the process. Identifying people with immune problems “is going to be hairy,” said William Schaffner, an infectious-diseases physician at Vanderbilt University and a liaison member to the CDC advisory panel. Older people have immune systems that typically weaken with age and may think they ought to be included, he said, but they are not the people officials are focusing on now.

Some doctors say prescriptions should be required — in part because they believe people should be tested for antibodies before and after the extra shots to see if the inoculations boosted immunity. While the FDA discourages use of antibody tests to gauge post-vaccination protection, some doctors have said the agency should reconsider its position for immunocompromised people, for whom such information is critical.

Dorry L. Segev, a transplant surgeon and researcher at Johns Hopkins University, said patients should talk to their doctors about the benefits and risks of a third shot. While extra doses might be beneficial, he said, they also stimulate the immune system, which could raise the risk a transplanted organ will be rejected or that an immune-related disease will flare up — though those risks are low.

“I hope that we see thoughtful caution in the pursuit of these boosters rather than a generic one-size-fits-all approach,” Segev said.

Studies have shown that immunocompromised people can benefit from an extra shot of a coronavirus vaccine. In a study published Wednesday in the New England Journal of Medicine, Canadian researchers found that a third dose of the Moderna vaccine bolstered protection for a substantial proportion of organ-transplant patients.

The study followed a smaller one co-written by Segev and published in June. It showed that people who had a small but detectable antibody response after being fully vaccinated got a big boost from an extra shot. But two-thirds of those who didn’t respond to the initial regimen had no change after the third dose.

Many immunocompromised people were delighted by the FDA action.

Christine Lipuma, a 54-year-old resident of Downers Grove, Ill., had been eagerly awaiting the chance to get a third dose. She has been on immune-suppressing drugs for years following a kidney transplant and had only a small increase in antibodies after getting two shots of the Moderna coronavirus vaccine this year, according to testing by Johns Hopkins as part of a study overseen by Segev.

Lipuma skips indoor dining and sees friends only outside. She considered getting a third shot on her own but decided to wait for the FDA to authorize it.

“I would like science behind the decision,” she said.

Jullie Hoggan, a 48-year-old kidney transplant patient who lives in Henderson, Nev., received two doses of the Pfizer-BioNTech vaccine in February and had no antibody response, according to the Hopkins study. She got a shot of the Johnson & Johnson vaccine at a mass vaccination clinic and still didn’t have an increase in antibodies.

Hoggan lives a highly restricted life. Just this week, her husband decided to move out of their house temporarily after attending a meeting with someone who tested positive for the coronavirus. “It is just so frustrating,” she said.

Hoggan doesn’t plan on getting another shot. Instead, she is trying to get Regeneron’s monoclonal antibody treatment under the company’s “compassionate use” program.

Ariana Eunjung Cha contributed to this report.

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