Skip to main content

Association between Universal Masking in a Health Care System and SARS-CoV-2 Positivity Among Health Care Workers

Our take —

This study compared the proportion of symptomatic health care workers (HCWs) testing positive for COVID-19 before and after universal masking policies were implemented within the health system. Before universal masking, the positivity rate was increasing rapidly, and following universal masking, the rate declined. Although other unmeasured factors could explain part of this change, the study suggests that universal masking was effective in reducing transmission.

Study design


Study population and setting

Between March 6 and April 29, 2020, 9,850 health care workers (HCWs) were tested for COVID-19 within the Mass General Brigham health system in the US state of Massachusetts. Two major masking interventions were implemented during this phase: on March 25, 2020, all HCWs were required to wear a mask; and on April 6, 2020, all patients were required to wear a mask. By tracking the proportion of HCWs that tested positive per day out of all tests conducted, the authors compared trends in positivity rates before and after universal masking policies were implemented.

Summary of Main Findings

Overall, 9,850 health care workers (HCWs) were tested for COVID-19. Before universal masking of HCWs and patients took place, the proportion testing positive per day was exponentially increasing from 0% to 21.32%. After implementation of universal masking for health care workers and patients, and allowing for a transition period, the positivity rate fell linearly from 14.65% to 11.46%. The decreasing positivity rate amongst HCWs after universal masking occurred even though case numbers for the state of Massachusetts were increasing.

Study Strengths

The authors used existing data coupled with changes in hospital policy to assess the impact of universal masking on the spread of COVID-19 amongst health care workers.


As the authors acknowledge, other state or local interventions and trends, such as physical distancing measures and uptake of masks by the general public, may have led to reduced case numbers amongst HCW. Declines in the positivity rate amongst HCWs occurred despite the case numbers increasing for Massachusetts, but no information was provided about trends in daily COVID-19 inpatients and outpatients treated within the health system. As testing was limited to those HCWs reporting COVID-19 symptoms, asymptomatic or pre-symptomatic cases were likely to be missed. It is also unclear which symptoms for COVID-19 were used to determine if a HCW should be tested. Even though the authors mention universal masking of all HCW and patients with surgical masks, it is unclear what proportion of HCW were using N95 masks, or powered and supplied air respiratory protection, and what proportion had direct patient contact. Finally, it is unclear if HCW were able to be tested more than once, given limited information on the sensitivity of diagnostic tests.

Value added

Given the paucity of data demonstrating the effect of masking on COVID-19 transmission, this study provides useful evidence about the effectiveness of universal masking of patients and health care workers.

This review was posted on: 24 July 2020