WASHINGTON—A variant of the coronavirus first identified in Britain is now the most common strain circulating in the United States. The director of the Centers for Disease Control and Prevention, Dr. Rochelle Walensky, says the strain, formally known as B.1.1.7, is “now the most common lineage circulating in the United States.” The strain has been shown to be more transmissible and infectious among younger Americans, which Walensky says contributed to rising case counts in recent weeks. Walensky says new outbreaks have been tied to youth sports and daycare centers. She particularly encouraged states with rising caseloads to curtail or suspend youth sport activities to slow the spread of the virus. The United States leads the world with 30.8 million confirmed cases and more than 556,000 confirmed deaths.