The research described in this article has been published as a working paper but has not yet been peer-reviewed by experts in the field. Masks reduce the spread of Covid-19. But just how much of an effect do they have? A study co-authored by an MIT professor finds that if the U.S. had introduced a uniform national mask mandate for employees of public-facing businesses on April 1, the number of deaths in the U.S. would likely have been 40 percent lower on June 1. “It is a very effective policy…
