The Food and Drug Administration will permit use of COVID vaccines by adults older than 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 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 FDA’s vaccine division chief, and Dr. Martin Makary, the agency’s commissioner, wrote in the New England Journal of Medicine.

“The FDA 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 age 6 months and older. Scientific advisers to the FDA 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 Prasad and 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, falsely claiming at one point that the COVID vaccines had killed more people than the virus.

As a measles outbreak spreads through Texas and other states this year, Kennedy has offered only muted support for vaccinations and has repeatedly raised questions about the safety of the measles shot while suggesting, falsely, that miracle treatments, such as cod liver oil, were available.

‘Reasonable compromise’

During a “town hall” livestreamed Tuesday afternoon, Prasad said he thought the new approach to COVID vaccination was “a reasonable compromise,” leaving the shots available to many Americans “but also generating evidence.”

Some Americans are worried that the FDA has “not fully documented and interrogated the safety harms” of the vaccines, while others “are desperate for additional protection,” he added, describing himself as an “extreme pro-vaccine person.”

Before approving COVID vaccines for wider use, the FDA “anticipates the need” for new clinical trials in which participants younger than 65 are randomly assigned to receive the new shots or a placebo, Prasad and Makary wrote in the journal.

Vaccine manufacturers should follow participants for at least six months and evaluate whether the effect of the vaccine persists over that time, Prasad said during the livestream.

Some experts said the new criteria go too far, erecting barriers for people who will want the shots.

“This is overly restrictive and will deny many people who want to be vaccinated a vaccine,” Dr. Anna Durbin, director of the Center for Immunization Research at Johns Hopkins University, said in an email.

Dr. Daniel Griffin, a physician in New York who has treated thousands of COVID patients, said, “I think that changes like this will lead to more unnecessary deaths.”

“What they’re really doing is they’re very slowly reducing vaccination in the country,” he said

Some experts said that while they agreed with aspects of the agency’s rationale, the new restrictions would unnecessarily limit access for healthy Americans who may be at high risk from COVID because of their jobs, or for other reasons.

This “doesn’t preserve people’s choice to get vaccinated,” said Dr. Denise Jamieson, vice president for medical affairs at the University of Iowa and an adviser to the Centers for Disease Control and Prevention.

Pfizer cites ‘robust data’

In a statement, Pfizer said it is evaluating the details shared Tuesday and is in ongoing discussions with the FDA. The company said its COVID vaccines have been given to more than 1 billion people, “generating robust data demonstrating a favorable safety profile.”

Moderna, maker of another widely used COVID vaccine, did not immediately comment.

The new recommendations mark a departure from the previous policy of offering annual COVID shots to everyone 6 months and older, the “most aggressive” strategy when compared with the standards used in other nations, the FDA officials noted.

Makary and Prasad have often been described as COVID contrarians. But recommending the vaccine for every American 6 months and older, regardless of health status, is in fact the “contrarian” approach, they said at the town hall.

“Now we are back in line with the rest of the world,” Prasad said. Other high-income nations have long adopted “risk-based” recommendations targeting the shots at those most in need.

The original vaccine trials were conducted years ago, at the onset of the pandemic. The virus was very different then, and most people had not yet been exposed. It’s unclear whether the shots would still prove as beneficial now, the FDA officials said.

Yet federal agencies do have robust data showing that, even now, COVID vaccines offer modest protection against hospitalization and death even at younger ages, some scientists noted.

“We need to also include the role of observational data and the role of the CDC in continuously monitoring vaccine safety and efficacy,” Jamieson said. “We have a lot of accumulated data at this point.”

But Prasad cast doubt on the quality of such data, saying those studies are “at high risk of bias and broadly uninformative.”

He and Makary also said there was insufficient evidence that getting the vaccine would prevent transmission of the virus to others at risk from the virus.

Deaths have fallen

Deaths from COVID have fallen each year since the start of the pandemic, but there were still roughly 1,000 deaths per week last winter, most of them in adults 75 and older.

In the year ending in August, the CDC reported 150 pediatric deaths, a number comparable to deaths among children in a typical flu season.

Children younger than 4 remain at high risk from COVID, but the absolute numbers of seriously affected children are low, the officials said, and children with medical conditions would still qualify for the vaccine.

It is the CDC that usually defines who should get a vaccine and when, based on guidelines set by a panel of external advisers. “CDC is the agency that really has the expertise in that area,” Jamieson said.

It is unclear how the new criteria will affect availability of COVID vaccines in the fall. The revised approvals process could upend insurance and public coverage of COVID vaccines, experts said.

The proposed regulations align with the agency’s recent decision to narrow the eligibility for the Novavax COVID vaccine.

The FDA on Friday approved that shot for use only in adults older than 65 and for others older than 12 who have at least one medical condition that puts them at high risk should they contract COVID.

The decision was met with some criticism, given the absence of any new evidence to suggest concerns about effectiveness or side effects.

It “does seems to be based more on this administration’s hostility to vaccines and resentment of COVID prevention methods,” Durbin said.