This week, the Centers for Disease Control and Prevention released new data that showed around 60 percent of Americans — more than half of adults and three quarters of children — have now been infected with the coronavirus. But herd immunity looks likely to remain elusive, and many people are still at high risk from Covid-19. What do the C.D.C. figures mean for immunity in the United States, and for the future of the pandemic? Guest: Apoorva Mandavilli, a science and global health reporter for The New York Times.