I’ve also heard a few people say, “I already got the vaccine, so I don’t need a mask.” This may be the most dangerous excuse of all. First, although the vaccines are very good, they’re not perfect, and chances are these vaccine recipients haven’t been checked for strong antibodies to the virus. Second, we don’t yet know if the vaccines, while highly effective in preventing sickness and death, will also prevent asymptomatic infection that can spread the virus to others.

As Jeremy Howard, a data scientist at the University of San Francisco, said of mask refusers: “How would you feel if you made your best friend sick, or killed your friend’s mother?”

Last February, after the World Health Organization, with no supporting data, advised against wearing a mask unless you were already sick, Mr. Howard amassed an international team of 19 scientists to review the evidence for mask-wearing, expecting to find “that masks were a waste of time,” he said in an interview. Instead, he said, the team found that “the data on the benefit of masks is really compelling.” The results of their exhaustive study were published recently in PNAS, the peer-reviewed Proceedings of the National Academy of Sciences.