Geert Vanden Wijngaert | Bloomberg | Getty ImagesA CDC advisory group is considering whether fully vaccinated Americans with weakened immune systems need a booster dose of a Covid vaccine after data shows they are less likely to have antibodies to fight the disease and more likely to suffer from a so-called breakthrough infection.Immunosuppressed populations represent 44% of hospitalized Covid breakthrough cases — an infection in a fully vaccinated individual, according to a slide presented Thursday at the agency’s Advisory Committee on Immunization Practices meeting. The population segment, which includes patients with cancer, HIV or those who have had organ transplants, represents only about 2.7% of the U.S. adult population, the presentation said. The Centers for Disease Control and Prevention’s advisory committee is also meeting to discuss the safety profile of the Johnson & Johnson vaccine.Immunocompromised people are more likely to become seriously ill from Covid and are at a higher risk of transmitting the virus to family and friends, the agency said.Studies suggest that a third vaccine dose might help patients whose immune systems don’t respond as well to a first or second dose. Four small studies cited by the CDC showed that 16% to 80% of people with weakened immune systems didn’t have detectable antibodies to fight Covid after two shots.Among immunosuppressed patients who had no detectable antibody response, 33% to 50% developed an antibody response after receiving an additional dose, according to the CDC.”Emerging data suggest that an additional COVID-19 vaccine dose in immunocompromised people enhances antibody response and increases the proportion who respond,” according to one slide presented at the meeting.The meeting comes as federal officials say booster doses of the vaccines for the general population are not needed at this time.The advisory panel cannot recommend additional shots for anyone until the Food and Drug Administration grants full approval of the Covid vaccines or amends the emergency use authorizations.Other countries, such as France, are already giving out third shots to people living with cancer or other immune impairments. The CDC group has previously said that more vulnerable Americans, such as elderly people or transplant recipients, may need an extra dose.Some doctors have been pushing for the U.S. to allow immunosuppressed populations to get an extra dose, according to Dr. Dan Barouch, an immunologist at Harvard Medical School who helped develop the J&J vaccine.”The hardest to vaccinate people are those who are immunosuppressed,” he said, adding early data shows a third shot may be safe and effective for those populations.— CNBC’s Rich Mendez and Robert Towey contributed to this article.