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A group of independent experts advising the CDC suggested there isn't enough data yet to support recommendations for COVID-19 booster vaccines, and expect to continue monitoring studies into the fall.

The advisory panel is specifically tracking potentially waning immunity over time, variants and any declines in vaccine efficacy, but Dr. Sara Oliver, member of the CDC's Epidemic Intelligence Service, said during the Advisory Committee on Immunization Practices (ACIP) meeting Wednesday that COVID-19 infection affords antibody protection for up to eight months, and mRNA vaccines revealed persistent antibody levels for up to six months post-second dose.

"There's not sufficient data to support recommendations for booster doses currently, but we'll continue to closely monitor," Oliver said.

Several experts urged the priority should be improving current vaccination rates in the face of concerning viral variants, which could in turn help protect the vulnerable, especially immunocompromised individuals and those in nursing homes. Another expert warned the U.S. isn't sending enough doses abroad to help suppress the pandemic globally.

Of note, the so-called "correlate of protection" is still unknown, or the immune response that helps scientists predict the degree of protection against infection or disease.

Nevertheless, growing evidence suggests immunocompromised individuals, who account for about 2.7% of the country's population, mount a diminished protective immune response, even after two doses of vaccine. For instance, 7-27 days after a second Pfizer dose suggested 75% protection against symptomatic COVID-19 among immunosuppressed patients, compared with 94% overall. The patients also saw less protection against infection at 71% versus 90% overall, after two Pfizer doses.

Separate findings suggested over half of some 30 organ transplant patients under study didn't respond to a third dose of vaccine. Experts said it was unclear whether additional doses would benefit this population, or other prevention measures, like monoclonal antibodies, could offer more protection.

The panel said more research is needed for mix-and-match booster series, or receiving a different vaccine than the primary series. Experts said they would continue to monitor breakthrough cases, severity of disease and genomic sequencing data, specifically for variants of concern.