Effectiveness of wearing a mask

Science has a lot to say about the effectiveness of wearing a mask to stop the spread of the coronavirus, but the communication of that science has been corrupted by a combination of partisan divides, sensationalist media stories, distrust, false dichotomies, and letting the perfect be the enemy of the good.
The studies on masks aren’t perfectly definitive, but that’s typical of many issues involving health risks — from mercury contamination to cancer screenings. It’s still better to make decisions based on an incomplete body of evidence than to ignore evidence altogether. In this case, it helps to add a dose of situational awareness and common sense.
The public health community got off on the wrong foot with masks by advising against wearing them and then making a sudden flip-flop and telling us not to leave home without them. Further complicating the picture is a mix of people’s individual attitudes. Some members of the public still fear the virus and want to remain safe, and others are at peace with their risk but want some guidance on how to be a good citizen — or at least be perceived as one.
There have been reasonably convincing studies showing that masks stop some of the particles that might carry the virus out of people’s mouths. That suggests masks’ potential to protect others. Then there are observational studies, which look at mask use in the real world.
On June 1, The Lancet published an analysis of 172 such studies, many of them done in health care settings. The authors concluded that mask-wearing combined with eye protection and social distancing could cut down on the spread of the virus, though the authors admitted to a high degree of uncertainty.
Another study came down on the side of mandatory mask-wearing by watching disease trends in Wuhan and New York City. But some other researchers noted flaws in that study, published in the Proceeding of the National Academy of Sciences. The one-to-two-week delay between infection and test results would suggest infections in New York City dropped well before masks were made mandatory. Some experts wanted the study retracted.
When there are multiple changes in behaviour going on at the same time, it can be impossible to connect any one of those changes to rising or falling case numbers. That doesn’t mean the information in those studies can’t be useful. Common sense would suggest that if an activity poses negligible risk, then wearing a mask offers only a negligible benefit, and should be optional.

—Bloomberg