Most people only need to wear high-filtration masks for short periods and in some circumstances. Outdoors, for example, there is no need to wear a mask unless you are in a crowd. But on poorly ventilated, crowded trains, cloth or no surgical masks are likely to be inadequate, potentially putting people at high risk of inhaling infectious aerosols. There are times when masks are more necessary than others, especially when community transmission rates are high, such as during the peak of the epidemic we are currently experiencing. This is true even for fully vaccinated people, in large part because our vaccines don’t stop infections the way they do severe illness and hospitalizations.