WASHINGTON (AP) -- The Trump administration's effort to impose new requirements on Novavax's COVID-19 vaccine -- the nation's only traditional protein-based option for the coronavirus -- is sowing uncertainty about updates to other vaccines, too.

Novavax said Monday 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."

But a weekend post on social media by FDA Commissioner Marty Makary 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 unusual move at FDA come shortly after the agency's longtime vaccine chief was forced out over disagreements with Makary's boss, Health Secretary Robert F. Kennedy Jr.

Kennedy won Senate confirmation to his job, in part, by promising not to change the nation's vaccine schedule. Since taking office, he's promised to "investigate" children's shots, canceled meetings of expert vaccine advisers and directed officials to look again for connections between vaccines and autism, a link long-ago debunked.

The Novavax vaccine, which originally showed effectiveness in a nearly 30,000-person clinical trial, is still being sold under emergency use authorization in the U.S. 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 FDA to make one change each year to their recipe -- which strain to target -- just like 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. But Trump appointees directed FDA scientists to pause their decision, according to one of the people. Since that highly unusual move, Novavax and the agency have been discussing additional requirements for approval.

In his weekend tweet, FDA's 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, in a statement.

The FDA had been treating the annual COVID-19 strain updates exactly as it's done for decades with flu vaccines -- not as new products, but existing ones that are merely adjusted to protect against the latest strains, said Dr. Paul Offit, a vaccine expert at Children's Hospital of Philadelphia.

Offit said the companies still must perform tests in small numbers of people that show these updated vaccines produce levels of virus-fighting antibodies known to be protective, and they're closely monitored for safety.

Nixon, the HHS spokesman, suggested the policy might not apply to the flu shot, "which has been tried and tested for more than 80 years."

Under federal law, the FDA is required to follow established procedures when issuing requirements to drugmakers for approval. If the agency skips certain steps or imposes additional requirements for political reasons, experts say, it could be sued by drugmakers -- or even patients, such as those who prefer the Novavax vaccine over its competitors because of an allergy or some other reason.

In addition to large clinical trials conducted before all three COVID-19 vaccines were cleared for use, there's data on real-world use, said former Health Secretary Xavier Becerra, who oversaw COVID-19 vaccine policy during the Biden administration.

"At the point where I had left, we had put some 700 million COVID vaccines into the arms of Americans," he said. "That's a pretty good size clinical trial."

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AP writer Amanda Seitz contributed to this story

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