The FDA plans to require new clinical trials for approval of annual COVID-19 boosters for healthy Americans under 65.

While details of the guidance remain closely held, some clues are likely found in the FDA’s late Friday letter approving Novavax’s Covid vaccine. Who can get it: The license covers a limited population — adults 65 and older and people ages 12 to 64 with at least one underlying condition putting them at high risk for severe disease — compared with the two messenger RNA shots available for Covid recommended for all people 12 and older.

FDA Commissioner Marty Makary and top U.S. vaccines regulator Vinay Prasad said based on data from tests that measure immune response in patients, they anticipate that the FDA will be able to approve the boosters for adults over the age of 65 years.