I'll begin with a  paradox , thusly stated: As Dr. Shira Doron, an epidemiologist at Tufts Medical Center,   puts it , “It is simultaneously true that masks work and mask mandates do not work.”   Further: To start with the first half of the paradox: Masks reduce the spread of the Covid virus by preventing virus particles from traveling from one person’s nose or mouth into the air and infecting another person. Laboratory studies have repeatedly demonstrated the effect. Given this, you would think that communities where mask-wearing has been more common would have had many fewer Covid infections. But that hasn’t been the case. In U.S. cities where mask use has been more common, Covid has spread   at a similar rate   as in mask-resistant cities. Mask mandates in schools also seem   to have done little   to reduce the spread. Hong Kong,   despite almost universal mask-wearing , recently endured one of the world’s worst Covid outbreaks. More: The main explanation seems to be that the ex