In mid-August 2021, the Centers for Disease Control and Prevention (C.D.C.) issued a recommendation for both vaccinated and unvaccinated Americans to begin wearing masks in public again, particularly in places experiencing outbreaks of COVID-19, driven by the Delta variant. Further compounding this concern is the lower propensity of unvaccinated individuals to wear masks. For example, a report from July 2021 found that unvaccinated Americans, on average, tended to wear masks less than vaccinated Americans by a margin of 25 percentage points.