The Centers for Disease Control and Prevention on Friday recommended a third dose of coronavirus vaccines for some immunocompromised individuals following the plan’s unanimous endorsement by an expert advisory panel.

The CDC’s backing paves the way for millions of moderately or severely immunocompromised people to get a third shot of either the Pfizer-BioNTech or Moderna vaccines as soon as this weekend. That means those 12 and older who are immunocompromised would be eligible for Pfizer-BioNTech; the Moderna vaccine is for those 18 and older.

The CDC recommendation, which follows the Food and Drug Administration’s decision Thursday night to amend its authorizations of the two vaccines to permit the third shots, “is an important step in ensuring everyone, including those most vulnerable to covid-19, can get as much protection as possible from covid-19 vaccination,” CDC Director Rochelle Walensky said in a statement.

As new coronavirus variants emerge and the longevity of vaccine protection remains unknown, scientists are researching how booster shots could work. (John Farrell/The Washington Post)

She noted that in small studies, fully vaccinated immunocompromised people have accounted for up to 44 percent of breakthrough cases. The agency is not recommending additional doses for any other groups at this time, she said.

The CDC will not require patients get a prescription or a letter from a doctor for the third shot, officials said. Consumers will have to attest only that they have impaired immune systems. The goal is to minimize barriers to vaccination, they said. Additional doses will be free, and people will be able to get them in the same way they got earlier shots — through pharmacies and other health-care providers.

The CDC, in a presentation to the advisory panel, detailed for the first time which patients should consider the extra shots. They include individuals being treated for tumors or cancers of the blood; organ transplant patients, including those have received stem cell transplants within the last two years; and those who have diseases that damage the immune system.

Also on the list are people with advanced or untreated HIV infection, those taking high-dose steroids and those who have chronic medical conditions that can weaken immune response, such as chronic kidney disease. The CDC estimates that about 2.7 percent of U.S. adults, about 7 million people, are immunocompromised.

The FDA’s authorization allowed extra doses of the two mRNA vaccines for organ transplant recipients and others with similar levels of immune impairment. The one-shot Johnson & Johnson vaccine was not included because there was insufficient data about the protection afforded by an additional dose, according to the agency.

Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, which oversees vaccines, cautioned the panel that additional doses appeared to be only “moderately effective” at bolstering protection for immunocompromised patients. He and CDC officials said such individuals should continue following social distancing precautions as well as other safety steps to reduce their chances of infection from the coronavirus.

Patients should also talk to their doctors about the severity and duration of their immune issues and whether they are taking medicine or doing other therapies that can lower their body’s response to vaccines.

Health officials and members of the Advisory Committee on Immunization Practices, repeatedly drew a distinction between third doses for the immunocompromised — considered part of the primary series for people who have failed to respond fully or at all — vs. boosters for people who have mounted an immune response. Officials are not recommending antibody testing to see if the inoculations boosted immunity. There are no FDA-approved tests to gauge post-vaccination protection.

Panel members stressed that the benefits of the third doses far outweighed the risks for these vulnerable patients. Immunocompromised patients are more likely to become seriously ill from covid-19, the illness caused by the coronavirus. They are also more likely to transmit the virus to household contacts and to have breakthrough infections. Once infected, they can also potentially harbor mutations that could lead to more virulent new variants, experts say.

The panel urged patients getting a third shot of the same vaccine as their first two doses, unless the same vaccine is unavailable.

Several members also discussed the urgency of sharing first doses with people around the world.

“We are all very worried about the Delta variant, but that may be the least of our problems if covid-19 continues to circulate across the U.S. and the world,” said Helen Keipp Talbot, an infectious-disease doctor at Vanderbilt University. “And so I think it’s incredibly important that we remember that we need to begin sharing more and more vaccines across the world.”

Studies have shown that many immunocompromised patients have a very weak or no responses to the vaccines, even after a full regimen. The FDA said it evaluated information on the use of a third dose of the Pfizer-BioNTech or Moderna vaccines in these individuals and determined that the administration of third vaccine doses may increase protection against the coronavirus.

The Pfizer-BioNTech vaccine is currently authorized for those age 12 and older, and Moderna is authorized for those 18 and older. The FDA authorizations for these vaccines have been amended to allow for a third dose at least 28 days following the two-dose regimen of the same vaccine for immunocompromised patients.

CDC officials posted additional detailed guidance for clinicians.

Panel members also heard a CDC presentation on the framework needed to evaluate the potential need for boosters. CDC will present more data over the next few weeks to assess whether boosters are necessary based on the durability of vaccine effectiveness and whether certain at-risk groups, including older Americans and health-care workers, should get them first. Israel has already started administering booster vaccines to older people, and other countries are planning to do so.

Biden administration officials appear increasingly convinced that boosters for the broader population will be necessary in the future. Officials are analyzing studies from Israel, Britain and pharmaceutical companies on the vaccines’ durability and which groups should get priority, according to federal officials who spoke on the condition of anonymity because they were describing internal discussions.

But panel members stressed that the best way to control the pandemic is for the unvaccinated to get their shots. “We have very little evidence at this point that waning immunity is a significant driver of disease,” said Matthew Zahn, medical director for epidemiology at the Orange County Health Care Agency in California. “Right now, the overwhelming driver of disease — serious disease in particular — is people who aren’t vaccinated.”

Discussions have become increasingly urgent as the virus’s delta variant, which now makes up more than 97 percent of all new infections, wreaks havoc in many parts of the country and some studies indicate a waning of the vaccines’ protection over time, those officials said. Many vaccinated Americans have sought out extra shots because they fear their current doses don’t offer sufficient protection against a virus once again roaring through all 50 states.

It’s not clear how many people may have taken matters into their own hands. For Pfizer and Moderna recipients alone, the CDC estimates that 1.1 million have received additional shots, according to an internal document reported by ABC News. On Thursday, Walensky warned that such decisions undermine the CDC’s safety monitoring since they make it harder to track who got what shots.

Alan Grossman, 64, a biology professor at the Massachusetts Institute of Technology, welcomed the authorization of the third shots for transplant patients like himself and said he is eagerly awaiting his turn.

Grossman, a heart transplant recipient, received his second shot of the Pfizer-BioNTech vaccine in February and has been trying to get a third vaccine dose. “I can’t do it without lying or making up a lot of stuff that I’m not willing to do,” he said.

Grossman said he thinks Massachusetts General Hospital, where he had his transplant operation in May 2006, will contact patients once the CDC gives final approval. “I would get in line as quickly as possible,” he said.