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

Novavax originally showed its vaccine was safe and effective in a 30,000-person clinical trial. The FDA had been on track to grant Novavax full approval – without restrictions -- 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.

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