In fact, there is strong evidence that masks do work to prevent the spread of respiratory illness. It just doesn't come from RCTs. It comes from Kansas. In July 2020 the governor of Kansas issued an executive order requiring masks in public places. Just a few weeks earlier, however, the legislature had passed a bill authorizing counties to opt out of any statewide provision. In the months that followed, COVID rates decreased in all 24 counties with mask mandates and continued to increase in 81 other counties that opted out of them.