The Centers for Disease Control and Prevention quietly updated its Covid-19 vaccine guidance Monday for large swaths of immunocompromised people, saying they can receive a booster dose of a Covid vaccine at least six months after completing their primary vaccination series. 

But unlike boosters for the vast majority of Americans, a booster shot for an immunocompromised individual may be their fourth Covid shot.

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Back in August, the CDC recommended that immunocompromised individuals who had been initially vaccinated with either the Pfizer-BioNTech or the Moderna mRNA vaccine ​​— but not Johnson and Johnson’s vaccine ​​— receive a third dose of an mRNA vaccine. That extra dose was not considered a booster, but rather a part of their primary vaccination series. 

The new booster dose can be any of the available vaccines, including the Johnson & Johnson vaccine, according to the CDC. 

Research has shown that people with compromised immune systems don’t mount an adequate immune response following vaccination, and the purpose of the third dose was to raise their immunity levels to what’s seen in people with normal immune systems after two doses. 

The goal of this fourth dose would be different: to combat waning immunity. It would serve the same purpose as a booster dose given to people without immune deficiencies six months after they were initially vaccinated. 

An estimated 2.7 percent of adults in the United States are immunocompromised, according to the CDC. That includes organ transplant recipients, certain cancer patients, and people with HIV.

“We know that six months after you reached a good level of protection, your protection has waned … and we need to boost that,” said Dr. Dorry Segev, professor of surgery at Johns Hopkins University, who is studying Covid vaccine responses in immunocompromised people. “That’s for people with normal immune systems and people who are immunocompromised.” 

But not every immunocompromised American will need a fourth shot.

“Out of the 11 million immunocompromised people in this country, some of them were fine with two doses,” Segev said. “Some of them were not fine with three doses. Some of them do need a fourth dose.”

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The change to the CDC’s clinical considerations comes less than a week after an advisory committee to the agency officially endorsed a booster shot for both the Moderna and the J&J Covid vaccines.

The question of how and whether to provide a booster dose to immunocompromised individuals who may have already received a third dose was brought up during the advisory committee’s meeting last Thursday.

“Under the EUA, someone who is immunocompromised and receives a third primary series dose would six months later be eligible for a booster dose,” Dr. Doran Fink, clinical deputy director of the division of vaccines and related products for the Food and Drug Administration, ​​told the committee.

In a statement to NBC News, CDC spokesperson Kristen Nordlund said, “The Clinical Considerations was updated as a result of that discussion.”

Dr. Balazs Halmos, director of thoracic oncology at the Montefiore Einstein Cancer Center in New York City, called the decision logical from a scientific standpoint but not necessarily data-driven.

Dr. Matt Kalaycio, the former chairman of the department of hematology and medical oncology at Cleveland Clinic, agreed. 

“I am unaware of data supporting a fourth dose in immunocompromised patients,” he said. “It is unclear to me on which data the CDC is leaning to reach their conclusion.” 

Still, because the third dose for immunocompromised patients was just authorized in August, and the booster is recommended six months after that, any fourth shot for this group is not “imminent,” Halmos said.

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