The Food and Drug Administration on Wednesday approved updated Covid vaccines for the fall season that limit who can get the shots, the federal government’s most restrictive policy since the vaccines became available.

The agency authorized the vaccines for people who are 65 and older, who are known to be more vulnerable to severe illness from Covid. Younger people would only be eligible if they have at least one underlying medical condition that put them at risk for severe disease, according to people notified of the approvals. Healthy children under 18 could still receive the shots if a medical provider is consulted.

People seeking the shots will soon face another hurdle. An influential advisory committee to the Centers for Disease Control and Prevention must vote to recommend them. But that panel’s makeup shifted when Health Secretary Robert F. Kennedy Jr. unseated existing members, reduced the panel’s size and added some Covid vaccine opponents.

This would mark the first fall/winter season that Covid shots were not widely recommended to most people and children, pitting federal health officials in the Trump administration against several national medical groups that oppose the restrictions. In a break from tradition, the F.D.A. did not issue a news release on the approvals.

Many public health experts view the changes as part of Mr. Kennedy’s broader campaign against certain vaccines, especially his targeting of mRNA technology, which has been used in the vast majority of shots administered to Americans. They criticized his recent cancellation of $500 million in grants to study flu and Covid vaccines, as a move that would significantly set back the nation’s efforts to develop better therapies and leave the nation reliant on older, slower approaches.

The F.D.A.’s new limited approval covers two vaccines designed with mRNA. People familiar with the details of the approvals said that the Moderna vaccine authorization covers those who are 6 months old and older and have medical conditions and all people over 65. The Pfizer shot was approved for the same group ages 5 and older.