A new, more transmissible variant of the virus that causes Covid-19 could sweep the United States in coming weeks and become the dominant strain as soon as March, leading to a new surge of cases through the spring, the CDC warned Friday. The CDC believes the variant, known as B117, is still circulating at low levels in the U.S. Only 76 infections caused by the new variant have been detected, in 12 states, though testing for it has not been routinely conducted. CDC officials acknowledge the variant is likely more widespread here than is currently recognized.