The Food and Drug Administration will permit use of Covid vaccines by adults over 65 and those with certain medical conditions in the fall, but may require additional studies before approving the shots for healthy Americans younger than 65, agency officials said on Tuesday.

At this point, the additional doses offer “uncertain” benefits to many young and middle-aged people who have already been vaccinated or have had Covid, Dr. Vinay Prasad, the F.D.A.’s vaccine division chief, and Dr. Martin Makary, the agency’s commissioner, wrote in The New England Journal of Medicine.

“The F.D.A. will approve vaccines for high-risk persons and, at the same time, demand robust, gold-standard data on persons at low risk,” the officials wrote.

Until now, annual Covid shots were recommended for everyone aged 6 months and older. Scientific advisers to the F.D.A. are set to meet Thursday to decide on the composition of the Covid vaccine to be made available in the fall.

During the pandemic, both Dr. Prasad and Dr. Makary sharply criticized vaccine mandates and other public health measures intended to turn back the coronavirus.

Robert F. Kennedy Jr., the health secretary, is a longtime vaccine skeptic who spent years campaigning against the Covid shots, claiming falsely at one point that the Covid vaccines have killed more people than the virus.

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