The FDA's approval was based on a study of 11,400 people age 12 and older that compared the new low-dose vaccine with Moderna's existing vaccine. It found the new vaccine was safe and was at least as effective - and more by some measures - than the original shot, the company said.

Moderna announced this weekend that the Food and Drug Administration approved its lower-dose Covid-19 vaccine for adults 65 and older, as well as people ages 12 to 64 with at least one medical condition that increases their risk for severe Covid.