When will it be safe to shop at a grocery store or show up at the office without wearing a mask? Sooner than most experts are willing to admit. If the coronavirus epidemic in the U.S. continues on its current trajectory, the need for masks outside particular local outbreak areas will pass in a matter of weeks. One way to think about the problem is by analogy to seasonal influenza. Hardly anybody wears a mask in ordinary settings to protect against the flu, and no one is required to do so. The worst flu seasons of recent years saw an...