The mandatory use of facemasks slows the growth in new cases of COVID-19 by 40 percent, according to a German study that provides the best evidence yet for their use. Tom Whipple writes in The Times that the research was able to use the staggered introduction of masks in shops and public transport across Germany as a natural experiment to test how effective they were. By looking at new cases in the days that followed, the researchers concluded that there is “strong and convincing statistical support” that the masks “strongly reduced the number of incidences.”