The Centers for Disease Control and Prevention is planning to update its guidance for some people with weakened immune systems to receive a booster dose of the coronavirus vaccine three months after completing the initial series of the Pfizer or Moderna vaccine rather than at the current interval of five months.

Agency officials presented the anticipated changes Friday during a meeting of the CDC’s outside vaccine experts on the Advisory Committee on Immunization Practices. The updated guidance is expected to be released Monday.

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Recommendations for some immunocompromised people are already different from guidance for the general population, because those with frail immune systems are at increased risk for severe COVID-19 complications and more likely to be infected even after they are fully vaccinated. Once infected, they can also potentially harbor mutations that could lead to more virulent new variants, experts say.

CDC recommends people with weak immune systems receive three primary shots plus a booster for a total of four doses. But given the less-than-optimal immune response that these individuals typically have with their first three doses, a booster at three months may provide better protection, especially when there is such high transmission of the omicron variant, Elisha Hall, a CDC health education specialist, said Friday.

The panel members welcomed the anticipated changes, saying they would provide helpful practical information for many of their patients.

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“In the past two months, I’ve seen many of these immunocompromised patients who had followed all the rules still have significant breakthrough infections. And I really think that this will help dramatically,” said Camille Kotton, a clinical director and infectious-disease specialist at Massachusetts General Hospital in Boston.

In addition to shortening the booster interval, officials are trying to address confusion about recommendations more broadly for this population because of recent reports that some people with weakened immune systems were being denied their fourth dose at some pharmacies.

The CDC estimates that about 2.7% of adults — 7 million people — are immunocompromised, a group that includes patients with medical conditions that weaken their immune response, as well as people taking immune-suppressing drugs because of organ transplants, cancer or autoimmune diseases. Also on the list are people with advanced or untreated HIV infections, and those who have chronic medical conditions that can weaken immune response, such as chronic kidney disease.

Hall said small studies in immunocompromised people showed an immune response when a fourth dose was given one to three months after the third dose. Multiple studies in the general population have also triggered an immune response following a booster as early as three months after a primary two-dose series. Many countries already give booster doses as early as three months in the general population, Hall said.

The agency also plans to revise guidance for people with weakened immune systems who received the one-shot Johnson & Johnson vaccine. The updated guidance will recommend an additional dose. That means two doses, at least 28 days apart, followed by a booster dose at least two months later, for a total of three shots.

Hall said many Johnson & Johnson recipients may have already received their primary dose and booster dose. The changes would, in effect, recommend a second booster.

The CDC will also be adding language to allow physicians to use their clinical judgment in giving mRNA vaccines — the Pfizer and Moderna shots — outside the dosing intervals approved by federal regulators.