The Centers for Disease Control and Prevention updated its guidance on Friday for some people with weakened immune systems, recommending they get a booster dose of the coronavirus vaccine three months after completing the initial series of the Pfizer or Moderna shots, rather than the current interval of five months.

The guidance also said immunocompromised people who received the one-shot Johnson & Johnson vaccine should get an additional dose. That means two doses, at least 28 days apart, followed by a booster dose of one of the mRNA vaccines.

“Although COVID-19 vaccines continue to work well to prevent severe illness, hospitalization, and death, we have seen reduced protection against mild and moderate disease,” the agency said in a statement. “With the number of cases of COVID-19 still high across the United States and globally, this guidance helps to ensure that people have optimal protection against” the virus that causes the disease.

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The updated guidance also says it is no longer necessary for people to delay coronavirus vaccination after receiving treatments with monoclonal antibodies, a treatment authorized for early stage COVID-19, or with convalescent plasma, which is taken from blood donated by people who have recovered from COVID-19.

Agency officials had presented the anticipated changes Feb. 4 during a meeting of the CDC’s outside vaccine experts on the Advisory Committee on Immunization Practices.

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

The CDC recommends people with weak immune systems receive three primary shots of the mRNA vaccines 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 amid high transmission of the omicron variant, officials have said.

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.

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