The Centers for Disease Control and Prevention on Friday encouraged people with weakened immune systems to come forward sooner for additional coronavirus shots, part of what the agency’s scientists described as a bid to shore up those people’s levels of protection in the face of the highly contagious Omicron variant.
Members of an advisory committee to the C.D.C. also signaled their support for another change to vaccination guidelines, indicating that they would endorse extending the gap between first and second doses of Pfizer-BioNTech or Moderna vaccines from roughly four to eight weeks.
That change, expected to reduce the risk of heart-related side effects and heighten the effectiveness of the vaccines, has not yet been voted on by the committee or implemented by the C.D.C.
Those and other updates to vaccination guidelines were the focus of a daylong meeting on Friday of C.D.C. scientists and expert advisers.
During the meeting, the advisers also updated their endorsement of Moderna’s coronavirus vaccine after the Food and Drug Administration on Monday granted full approval to the shot, making it the second to receive full regulatory approval. Dr. Rochelle P. Walensky, the C.D.C. director, gave her own backing to Moderna’s vaccine later on Friday.
Much of the C.D.C. meeting centered on how to better protect people with weakened immune systems, many of whom produce fewer antibodies in response to a vaccination or an infection, leaving them susceptible to the virus and to higher risks of serious illness. Some immunocompromised people have lately complained that pharmacies or hospitals have refused them additional vaccine doses recommended by the C.D.C.
C.D.C. scientists said they hoped that the new guidance would smooth that group’s path to receiving additional shots.
For people with moderately or severely weakened immune systems who had received the recommended three doses of the Pfizer-BioNTech or Moderna vaccines, the C.D.C. shortened the period that they had to wait for a fourth dose, reducing it from five months to three.
Dr. Elisha Hall, a C.D.C. scientist, said that the agency was concerned about waning levels of protection in people with weakened immune systems, especially given how quickly the Omicron variant spreads. Dr. Hall said that small studies had also shown that administering a fourth dose shortly after a third still strengthened immune responses in that group.
Federal regulators in August authorized a third Pfizer-BioNTech or Moderna dose for some immunocompromised people who had originally received those shots. Health authorities considered the additional dose for this group an integral part of their primary immunization series, rather than a booster shot.
The C.D.C. on Friday also encouraged people with moderately or severely weakened immune systems who had originally been given the Johnson & Johnson vaccine to take two additional vaccine doses, rather than only one.
The C.D.C.’s expert advisers applauded the push to better protect people with weakened immune systems.
Dr. Camille Kotton, an infectious disease physician at Massachusetts General Hospital, said that she had seen a number of immunocompromised people experience “significant breakthrough infections” during the Omicron surge. “I really think this will help dramatically,” she said.
The C.D.C. also gave doctors leeway to administer Pfizer-BioNTech or Moderna vaccines to immunocompromised people outside of the recommended dosing intervals in cases where “the benefits of vaccination are deemed to outweigh the potential and unknown risks.”
In a separate update, the C.D.C. did away with restrictions on when Covid patients who had been treated with monoclonal antibody infusions could be vaccinated.
The C.D.C. had previously recommended that people given the antibodies — a Covid treatment generally administered intravenously at hospitals or clinics — wait 90 days before being vaccinated with either a primary or a booster dose.
But the updated C.D.C. guidance said that those patients no longer needed to wait, based on evidence that people who had recently received the treatment still responded well to a vaccine.
It is not clear when the C.D.C. could take action on lengthening the time between first and second doses of the Pfizer-BioNTech and Moderna vaccines.
C.D.C. scientists on Friday presented evidence that an eight-week interval resulted in 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. (Those conditions are typically mild, and can also arise from Covid-19.)
Dr. Matthew Daley, a senior investigator at Kaiser Permanente in Colorado and a member of the C.D.C. advisory committee, said he hoped the change would persuade some people to get vaccinated. “If the message is, ‘We already have a highly effective and highly safe vaccine or vaccines, and this is an approach to make them even safer,’ that might convince some folks.”