Scientists at the US Food and Drug Administration said on Tuesday that Moderna Inc had not met all of the agency's criteria to support the use of booster doses of its Covid-19 vaccine, possibly because the efficacy of the shot's first two doses has remained strong.

FDA staff said in documents that data for Moderna's vaccine showed that a booster shot does increase protective antibodies, but the difference in antibody levels before and after the shot was not wide enough, particularly in those whose levels had remained high...