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Credit…Callaghan O’Hare for The New York Times

The Centers for Disease Control and Prevention on Friday encouraged people with weakened immune systems to come forward early for additional coronavirus injections, in what agency scientists described as an attempt to bolster the levels of protection these people have against the highly contagious Omicron. variant.

Members of a CDC advisory committee also signaled their support for another change to vaccine guidelines, saying they would approve of extending the gap between the first and second doses of Pfizer-BioNTech or Moderna vaccines. about four to eight weeks.

This change, which is supposed to reduce the risk of cardiac side effects and increase the effectiveness of vaccines, has not yet been voted on by the committee or implemented by the CDC.

These and other updates to the vaccine guidelines were the subject of an all-day meeting Friday of CDC scientists and expert advisers.

During the meeting, advisers also updated their approval of Moderna’s coronavirus vaccine after the Food and Drug Administration granted full approval for the vaccine on Monday, making it the second to receive approval. full regulations. CDC director Dr. Rochelle P. Walensky gave her own backing for Moderna’s vaccine later Friday.

Much of the CDC meeting focused on how to better protect people with weakened immune systems, many of whom produce fewer antibodies in response to a vaccination or infection, leaving them susceptible to the virus and at higher risk of serious illness. Some immunocompromised people have recently complained that pharmacies or hospitals have refused them extra vaccine doses recommended by the CDC.

CDC scientists said they hope the new guidelines will ease this group’s path to receiving additional injections.

For people with moderately or severely compromised immune systems who have received the recommended three doses of the Pfizer-BioNTech or Moderna vaccines, the CDC has shortened the waiting period for a fourth dose from five months to three.

Dr. Elisha Hall, a CDC scientist, said the agency is concerned about declining levels of protection in people with weakened immune systems, especially given how quickly the Omicron variant spreads. Dr Hall said small studies had also shown that giving a fourth dose soon after a third further boosted immune responses in this group.

In August, federal regulators authorized a third dose of Pfizer-BioNTech or Moderna for certain immunocompromised people who initially received these injections. Health authorities considered the extra dose for this group to be part of their primary vaccination series, rather than a booster shot.

On Friday, the CDC also encouraged people with moderately or severely compromised immune systems who originally received the Johnson & Johnson vaccine to take two additional doses of the vaccine, rather than just one.

CDC expert advisers have applauded the move to better protect people with weakened immune systems.

Dr. Camille Kotton, an infectious disease physician at Massachusetts General Hospital, said she saw a number of immunocompromised people suffer “significant breakthrough infections” during the Omicron surge. “I really think that will help tremendously,” she said.

The CDC has also given physicians leeway to administer Pfizer-BioNTech or Moderna vaccines to immunocompromised individuals outside of recommended dosing intervals in cases where “the benefits of vaccination are judged to outweigh the potential and unknown risks.”

In a separate update, the CDC removed restrictions on when Covid patients who had been treated with monoclonal antibody infusions could be vaccinated.

The CDC had previously recommended that people receiving the antibodies — a Covid treatment usually given intravenously in hospitals or clinics — wait 90 days before being vaccinated with a primary dose or a booster dose.

But updated CDC guidelines said those patients need not wait any longer, based on evidence that people who had recently received the treatment still responded well to a vaccine.

It’s unclear when the CDC might take action to extend the time between the first and second dose of the Pfizer-BioNTech and Moderna vaccines.

On Friday, CDC scientists presented evidence that an eight-week interval leads to stronger immune responses and higher levels of protection against infection or hospitalization. They also said it reduced the risk of serious but rare side effects seen especially in young men – myocarditis, or inflammation of the heart muscle, and pericarditis, or inflammation of the lining around the heart. (These conditions are usually mild and can also result from Covid-19.)

Dr. Matthew Daley, principal investigator at Kaiser Permanente in Colorado and a member of the CDC’s advisory board, said he hopes the change will persuade some people to get vaccinated. “If the message is, ‘We already have one or more very effective and very safe vaccines, and this is an approach to making them even safer,’ that might convince some people.”