More than 100 prominent health officials have signed an open letter urging governors to make wearing cloth masks in public mandatory, citing a “preponderance of evidence” they greatly reduce the transmissibility of COVID-19. The letter was written by Jeremy Howard, a research scientist who has studied the efficacy of masks at length. Research suggests mask-wearing could effectively halt the spread of the virus ― but only if at least 80%, and ideally all people, wear them.