WASHINGTON (AP) — The Trump administration has made an effort to impose new requirements on Novavax’s COVID-19 vaccine, the nation’s only traditional protein-based option for the coronavirus.

Novavax earlier this week said that the Food and Drug Administration was asking the company to run a new clinical trial of its vaccine after the agency grants full approval.

The company said it had responded and that it believed its shot remains “approvable.”

A weekend post on social media by FDA Commissioner Marty Makary, however, suggested the prospect of needing a new trial before the shots’ yearly strain update — something unlikely to be possible before fall. That’s raised questions about whether other vaccines will be caught in the turmoil.

“I don’t think because there’s a strain change that this is a new product,” said Dr. Jesse Goodman of Georgetown University, a former FDA vaccine chief. If that’s the new policy, “you’d always be doing clinical trials and you’d never have a vaccine that was up to date.”

The move at the FDA came shortly after the agency’s longtime vaccine chief was forced out over disagreements with Makary’s boss, Health Secretary Robert F. Kennedy Jr.

Since taking office, Kennedy has promised to “investigate” children’s shots, canceled meetings of vaccine advisers and directed officials to look again for connections between vaccines and autism.

The Novavax vaccine, which originally showed effectiveness in a nearly 30,000-person clinical trial, is still being sold under emergency use authorization. The nation’s other two options, mRNA vaccines made by Pfizer and Moderna, have earned full FDA approval for certain age groups.

Because the coronavirus continually mutates, manufacturers follow instructions from the FDA to make one change each year to their recipe — which strain to target — similar to flu vaccines.

The FDA was on track to grant Novavax full approval by its April 1 target date, according to two people with direct knowledge of the situation who spoke on condition of anonymity to discuss confidential agency matters. Trump appointees, however, directed FDA scientists to pause their decision, according to one of the people.

Since that move, Novavax and the agency have been discussing additional requirements for approval.

In his tweet, Makary referred to the Novavax vaccine as “a new product,” presumably because it had been updated to match last year’s common coronavirus strain.

“New products require new clinical studies,” Makary added.

An administration spokesman didn’t respond to specific questions about Pfizer and Moderna but suggested all COVID-19 vaccines could face stricter requirements.

“It’s now been years since COVID has presented the threat it once did and the urgency to rush approval of boosters without normal oversight no longer exists,” said Andrew Nixon, a Health and Human Services spokesman.