After a six-week delay, the US Food and Drug Administration has approved Novavax’s Covid-19 vaccine, according to a letter from the agency, but only for people 65 and older and those 12 and up who have at least one underlying condition that puts them at higher risk of severe illness.

The agency narrowed who can get the shot and added new study requirements that could cost the company tens of millions.

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.