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.

Over the weekend, the FDA issued final approval for a new type of vaccine from Moderna, a next generation shot that allows for a lower dose by refining its targeting system, but the FDA will only allow it for seniors and at-risk groups.