For the study, the researchers compared the rate of COVID-19 cases between the Cambridge Health Alliance (CHA) and Massachusetts residents from March 17 to May 6. The CHA started requiring masks on March 26. The universal masking was done during a time of rising infections in both the health care system and the community. After the policy was instituted, infections among health care workers sharply decreased, while community infections continued to rise until their peak on April 20. Before the mask mandate, infection rates increased almost identically in the health care system and the state's population, the study found.