Novavax had expected full approval for broader use by April 1, after the FDA reviewed data from a 30,000-person clinical trial showing the vaccine was safe and effective. But instead of a broad approval, the FDA asked the company to conduct more research.

The US Food and Drug Administration is changing the way it approves Covid-19 vaccines for Americans, a move that may limit future shots to older Americans and people at higher risk of serious Covid-19 infection.

The FDA plans to require new clinical trials for approval of annual COVID-19 boosters for healthy Americans under 65.

"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 agency said in an article published by The New England Journal of Medicine, authored by FDA Commissioner Dr. Martin Makary and his new top vaccines official, Dr. Vinay Prasad.

WASHINGTON — Robert F. Kennedy Jr. clinched the political support needed to become the nation’s top health official by pledging to work within the decades-old federal system for approval and use of vaccines. Yet his regulators are promising big changes that cloud the outlook for what shots might even be available.