The new memos from the Food and Drug Administration show how the agency’s vaccine chief, Dr. Vinay Prasad, personally intervened to place restrictions on COVID shots from vaccine makers Novavax and Moderna.

Both vaccines were approved by the FDA in May after months of analysis by rank-and-file FDA reviewers.

But internal correspondence show Prasad disagreed with staffers who planned to approve the shots for everyone 12 and older, similar to previous COVID vaccines. The scientists had concluded the benefit from the vaccines and the risk of COVID-19 outweighed the risk of possible side effects, which are rare.

Instead Prasad decided the shots should be limited to those who face special risks from the virus— seniors or children and adults with underlying medical issues.

Prasad explained that the COVID vaccine benefits must be reconsidered in light of falling rates of death and hospitalization and the possibility for vaccine side effects. It’s the latest in a series of vaccine restrictions imposed by officials working under Kennedy, who has long questioned the benefits of vaccines.

“Even rare vaccination related harms both known and unknown now have higher chance of outweighing potential benefits” Prasad wrote in a five-page memo explaining his decision.

COVID-19 remains a public health threat, resulting in 32,000 to 51,000 U.S. deaths and more than 250,000 hospitalizations since last fall, according to the Centers for Disease Control and Prevention. Most at risk for hospitalization are seniors and children under 2 — especially infants under 6 months.

Top FDA leaders are typically not involved in the review of individual products. Officials like Prasad can overrule staffers, but such cases are rare and often controversial.

News of the FDA documents was first reported by the New York Times.

Prasad was hired to lead the FDA’s vaccine center in May, after the previous director, Dr. Peter Marks, was forced to resign over disagreements with Kennedy. An academic researcher specializing in cancer therapies, Prasad came to prominence during the pandemic for criticizing public health measures, including the FDA’s approval of COVID boosters for healthy adults and children.

Since arriving at the agency he has worked with FDA Commissioner Mark Makary on new guidelines that will limit approvals of future COVID boosters to higher-risk Americans, mainly seniors and those with medical conditions like asthma and obesity.

Those limits match the terms FDA recently approved for Novavax’s shot, Nuvaxovid and Moderna’s mNexspike. Novavax’s vaccine is the only protein-based coronavirus vaccine available in the U.S. Moderna’s vaccine is an updated, lower-dose version of its existing mRNA-based vaccine.

The review team for the Novavax vaccine pointed to data from a study in 30,000 adults, concluding that “the risk-benefit assessment for this vaccine technology remains favorable.”

FDA staff reached a similar conclusion for the Moderna vaccine, deeming it similar in safety and effectiveness to the company’s original shot.

Last week, the FDA finalized new warning labeling about the risk of myocarditis, a rare form of heart inflammation, on shots from Moderna and Pfizer, the other maker of an mRNA-based shot for COVID.

In his “override memo,” reversing FDA staff’s decision on the Moderna shot, Prasad pointed to the ongoing risk of myocarditis and questions about its frequency. The agency ordered Moderna to conduct further studies of the risk as a condition for the approving its updated shot.

A spokesman for the administration said Prasad “has raised serious concerns” about the issue.

“We will not ignore these risks and will ensure that the gold standard of science is used for any decisions,” said Andrew Nixon, in an emailed statement.

Outside researchers have noted that cases of the heart condition tend to resolve quickly and are less severe than those associated with COVID infection itself, which can also cause myocarditis.