Starting next year, some immunocompromised people may receive a fourth Covid vaccine dose, the C.D.C. says.

Some American adults with weakened immune systems who received a third dose of either the Pfizer-BioNTech or Moderna coronavirus vaccine authorized just for them will become eligible for a fourth shot as a booster next year, according to updated guidelines from the Centers for Disease Control and Prevention.

“In such situations, people who are moderately and severely immunocompromised may receive a total of four vaccine doses,” with the fourth coming at least six months after the third, the C.D.C.’s guidelines said.

In August, federal regulators cleared a third dose of the Pfizer-BioNTech and Moderna vaccines for some immunocompromised recipients of those vaccines, instructing them to get it at least 28 days after their second shot. Federal agencies said that studies have shown that those people may not be adequately protected by just two shots.

The earliest that immunocompromised people who received that third mRNA vaccine shot can get a fourth shot as a booster would be February. The agency said that people could select that booster from any of the three coronavirus vaccines available in the United States.

The C.D.C. also recommends that moderately and severely immunocompromised adults who received Johnson & Johnson’s one-dose vaccine get another dose of any one of the three vaccine brands, at least two months after their initial shot.

The agency updated its guidelines on Monday, adding the possibility of a booster dose for many immunocompromised people, including those undergoing chemotherapy, recovering from a solid organ transplant or facing certain other medical issues, like infection with H.I.V.

The new recommendations also specified that a fourth dose of Moderna’s vaccine should be half the size of a normal dose.

Many health officials and experts in the United States and other countries have made a distinction between additional shots for immunocompromised people, who may not have mounted a strong immune response after their initial doses, and broader booster programs intended to shore up other peoples’ immunity, which can wane against infection naturally over time.

The World Health Organization has supported additional doses for people with weakened immune systems while calling for a global moratorium until the end of the year on booster programs for otherwise healthy people, so that more doses can be allocated to lower-income countries with low rates of vaccination.

The call for a moratorium has not stopped countries like Israel, the United States and Germany from moving ahead with booster programs.