> In this issue of JAMA, Wang et al present evidence that universal masking of health care workers (HCWs) and patients can help reduce transmission of severe ac

> In this issue of JAMA, Wang et al present evidence that universal masking of health care workers (HCWs) and patients can help reduce transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. > In the largest health care system in Massachusetts with more than 75 000 employees, in tandem with routine symptom screening and diagnostic testing of symptomatic HCWs for SARS-CoV-2 infection, leadership mandated a policy of universal masking for all HCWs as well as for all patients. > The authors present data that prior to implementation of universal masking in late March 2020, new infections among HCWs with direct or indirect patient contact were increasing exponentially, from 0% to 21.3% (a mean increase of 1.16% per day). > However, after the universal masking policy was in place, the proportion of symptomatic HCWs with positive test results steadily declined, from 14.7% to 11.5% (a mean decrease of 0.49% per day). > Although not a randomized clinical trial, this study provides critically important data to emphasize that masking helps prevent transmission of SARS-CoV-2.