There is not sufficient evidence to recommend more than one COVID-19 booster shot a year for older people and those with weakened immune systems, an expert advisory group to the U.S. Centers for Disease Control and Prevention (CDC) said on Friday.

The COVID-19 working group of the CDC's Advisory Committee For Immunization Practices (ACIP) supported an annual booster campaign, likely in the fall, especially for populations considered at high risk, Dr. Sara Oliver, a CDC official who heads the group, said during a meeting of the agency's outside advisers.

The agency currently recommends older and immunocompromised people receive COVID booster shots more frequently since vaccine effectiveness usually wanes faster for those populations compared to younger people with robust immune systems.

In the spring of 2022, the CDC recommended immunocompromised and people over age 50 receive an additional shot if they had received their first booster at least four months earlier.

The CDC advisers did not vote on new recommendations for how the COVID-19 shots should be administered on Friday.

But ACIP advised showing flexibility in recommendations for those with compromised or weakened immune systems to allow more frequent doses for those most vulnerable to severe COVID.

Both the CDC and the U.S. Food and Drug Administration are working on how to best update COVID vaccines to target circulating variants annually, similar to flu vaccine campaigns.

About 53.3 million people in the United States - around 16% of the U.S. population - have received a COVID-19 booster shot since updated versions of the vaccines were authorized in September.

That compares with 230 million people, around 70% of the population, that received an initial two-dose series of the COVID vaccines.