Moderna, over the weekend, announced that the U.S. Food and Drug Administration had approved its second-generation COVID vaccine, but only for specific populations: all adults 65 and over and those between 12 and 64 with at least one underlying risk factor as defined by the Centers for Disease Control and Prevention.

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.