“Herd immunity” has been a hot topic since day one of the pandemic. The term refers to a certain percentage of people that must be immune in order to block the spread of a virus. Experts used to say that 60% or more of the population would need to be immune to the novel coronavirus in order to help curb the spread. But some experts now say that this number might be lower. This is good news, but it comes with caveats. In his weekly column The Nuance, Markham Heid writes about the latest silver linings and limitations of Covid-19 immunity.