WASHINGTON – The Centers for Disease Control and Prevention is exploring an option to get additional coronavirus doses to immunocompromised people through a program that allows sick people to get unapproved drugs, a senior vaccine official said Thursday.

At a meeting Thursday of its advisory panel on vaccines, panel members spoke of the urgent need to give additional doses to people with fragile immune systems amid growing concerns about waning immunity in vulnerable populations and surging infections from the delta variant.

But the Advisory Committee on Immunization Practices cannot recommend that until the Food and Drug administration gives full approval to the currently available vaccines, or amends its emergency use authorization. The FDA’s existing authorization permits only a two-dose regimen of the Pfizer-BioNTech and Moderna vaccines, or a single dose of the Johnson & Johnson product.

Immunocompromised patients represent about 3% of U.S. adults, and include those who are organ transplant recipients, people on cancer treatments, and people living with rheumatologic conditions, HIV and leukemia. 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.

Amanda Cohn, CDC’s chief medical officer for vaccine policy, said the agency is exploring options to make additional doses possible for such patients.

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“I think what you’re asking about is, you know, is there a way to offer a third dose to individuals . . . through a study, or through an investigational new drug format for this population? I will just say that we are actively looking into ways that could be done to potentially provide access earlier than any potential change in regulatory decisions,” Cohn told panel members.

When clinicians and others want to use a product that is not FDA-approved, they can seek an investigational new drug application from the FDA, under a program known as expanded access, or more commonly, compassionate use. It gives patients with a serious disease or condition a potential pathway to get an investigational product. The coronavirus vaccines are considered such products.

This route is one option CDC is exploring as a bridge until regulators give full approval to the vaccines, a CDC official said. Such a process would require enrolling individuals in a clinical study where additional doses can be given.

Camille Kotton, a transplant-medicine physician at Massachusetts General Hospital in Boston and a member of the advisory committee, said many patients have “taken matters into their own hands and many are proceeding with additional doses of vaccine as they see fit,” which she said is “really challenging.” She said she is concerned about patients doing this in an unsupervised fashion, noting “our hands are really tied with the current regulatory situation.”

The situation also raises equity concerns, she said because patients who tend to be more educated and more empowered to take care of their own health care are getting additional doses, while others are being left behind.

The CDC used the expanded access program in 2013, for instance, to allow a vaccine that was not yet licensed to be given to students at increased risk for meningococcal disease during an outbreak at college campuses.

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Emerging data suggest that an additional dose in immunocompromised people may enhance disease-fighting antibody response and increase the proportion of those who respond to the vaccines, Sara Oliver, a CDC vaccine expert, told panel members.

On June 23, the New England Journal of Medicine published results of a study of 101 organ transplant patients in France who received three doses of the Pfizer-BioNTech vaccine and found their rate of antibodies against the coronavirus was 4% after the first dose, 40% after the second dose and 68% after a third dose – showing the third dose dramatically increased the likelihood of developing a disease-fighting response.

For people who showed no protection after the second dose, 44% developed an antibody response after the third dose, according to the study.

Debate over the need for booster shots has intensified since Pfizer and BioNTech recently announced they would seek regulatory approval for a third inoculation for all eligible adults, not just the immunocompromised, amid rising global concern about the highly transmissible delta variant. The companies’ announcement that they would seek an emergency-use authorization for a booster prompted health officials to declare that Americans who have been fully vaccinated do not need a third shot at this time.

But health officials make a distinction between boosters for people who already have an immune response after getting vaccinated, versus additional doses for the immunocompromised who have failed to respond fully or at all to the shots.

At a Senate panel hearing earlier this week, Anthony Fauci, the White House’s chief coronavirus medical adviser and director of the National Institute of Allergy and Infectious Diseases, said immunosuppressed individuals “may actually need a boost as part of their initial regimen, in the sense of getting them up to the point where they are protected.”

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Some panel members and the World Health Organization have also criticized the attention on boosters when so much of the world has little access to first doses. Low global vaccination rates and the spread of variants elsewhere in the world pose a risk for people in the United States.

Israel’s Ministry of Health on July 12 began offering a third dose of the Pfizer-BioNTech coronavirus vaccine to severely immunocompromised adults. Britain has announced a plan for booster shots to begin in September, targeted first to groups that include people who are immunosuppressed, people older than 70 and front-line health-care workers.

Also at the meeting, the CDC panel also heard additional information Thursday about an FDA warning issued July 12 about a serious but rare autoimmune disorder that has been linked to the Johnson & Johnson coronavirus vaccine. About 100 preliminary cases of Guillain-Barré syndrome, in which the immune system attacks the nerves, have been detected in vaccine recipients after the administration of 12.8 million shots in the United States, according to the CDC, which monitors vaccine safety systems with the FDA. Of those cases, 95 were serious and required hospitalization, with one death, according to the FDA.

The fatality was in a 57-year-old man with a history of heart failure, stroke, hypertension and diabetes, who presented with weakness five days after his Johnson & Johnson vaccination and was later hospitalized, according to slides prepared by the FDA for the meeting. The man died 25 days after getting vaccinated.

Such cases have largely been reported about two weeks after vaccination and mostly in older men, with a median age of 57, according to a vaccine safety monitoring system that tracks reports but does not assess causation. Most people fully recover from Guillain-Barré.

Available data do not show an increased risk with the vaccines made by Moderna or Pfizer-BioNTech. More than 325 million doses of those two-dose vaccines – developed with different technology than what is used to make the Johnson & Johnson product – have been administered in the United States.

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After detailed presentations summarizing the benefits and risks of all three vaccines, panel members said they strongly supported their continued use, noting the benefits far outweighed the risks.

One slide summarized those benefits: For every 1 million doses of Johnson & Johnson administered to women aged 18 to 29, 700 hospitalizations for covid-19 were prevented, compared to one new case of Guillain-Barré and four to five cases of rare blood clots linked to the vaccine. The benefits of the Johnson & Johnson product rose dramatically for older men, preventing 11,800 covid-19 hospitalizations, compared seven or eight cases of Guillain-Barré and no cases of blood clots.

For the Pfizer and Moderna vaccines, for every million doses given, the vaccines prevented 750 hospitalizations compared to the risk of three to four cases of the rare heart condition known as myocarditis.

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The Washington Post’s Laurie McGinley contributed to this report.