Health officials are tracking the spread of a number of variants of the coronavirus across the U.S., including a much more contagious version first identified in South Africa, another that's rampant in the U.K., and one linked to travelers from Brazil. Citing preliminary studies, the CDC said the Pfizer and Moderna vaccines show signs of recognizing those variants. South Africa suspended the start of its Oxford-AstraZeneca inoculation program in early February over concerns the shot doesn't work as well against the variant there.