Memos released in recent days by the Food and Drug Administration show that the agency’s vaccine chief overruled staff scientists who favored widespread access to Covid shots, setting off a firestorm of criticism from lawmakers, state officials and doctors.

Agency staff members had concluded that the F.D.A. should allow a wide range of age groups to receive the vaccines, citing high hospitalization rates among young children with Covid and saying that the virus’s evolution is “complex and remains unpredictable.”

But Dr. Vinay Prasad, the agency official in charge of vaccines and gene therapies at the F.D.A., disagreed, overriding those scientists and deciding to issue very narrow eligibility limits.

The F.D.A. last week said that no one under 65 was eligible to receive the Covid vaccines made by Moderna or Pfizer unless they had underlying medical conditions that put them at risk for severe disease.

The agency’s policy was the most restrictive to date for public access to the shots, intensifying opposition to the Trump administration’s retreat from longstanding vaccine standards and accepted childhood immunization schedules.

The fissure underscores the continuing clashes between Health Secretary Robert F. Kennedy Jr.’s decision-making on vaccine policy and the positions of long-serving federal officials and public health experts. The split was most recently exemplified by the mass exodus last week of top officials at the Centers for Disease Control and Prevention.