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.

The FDA has approved the Novavax COVID-19 vaccine, called Nuvaxovid, mainly for adults 65 or older, but people ages 12 to 64 with medical conditions that increase their risk of serious illness from COVID can also get the shot.

The FDA has approved Moderna’s new Covid-19 vaccine, though it placed restrictions on its use that the company’s existing Covid shot does not currently face.