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 U.S. Food and Drug Administration approved Novavax's COVID-19 vaccine with age restrictions after a six-week delay.

The notice came shortly after an advisory panel meeting at which members debated the pros and cons of asking manufacturers to update their vaccines.