Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention (CDC), adjusts her protective mask during a Senate Health, Education, Labor, and Pensions Committee hearing in Washington, D.C., U.S., on Thursday, March 18, 2021. Dangerous coronavirus variants continue to threaten progress the U.S. has made in reducing Covid-19 cases and immunizing the population, according to the nation’s top infectious disease doctor. Photographer: Susan Walsh/AP/Bloomberg via Getty Images