Swedish authorities have not officially declared a goal of reaching herd immunity, which most scientists believe is achieved when more than 60 percent of the population has had the virus. But augmenting immunity is no doubt part of the government's broader strategy?"or at least a likely consequence of keeping schools, restaurants, and most businesses open. Anders Tegnell, the chief epidemiologist at Sweden's Public Health Agency, has projected that the city of Stockholm could reach herd immunity as early as this month. Based on updated behavioral assumptions (social-distancing norms are changing how Swedes behave), the Stockholm University mathematician Tom Britton has calculated that 40 percent immunity in the capital could be enough to stop the virus's spread there and that this could happen by mid-June.