Universal masking lowers COVID-19 infections among healthcare workers, Harvard study finds

Universal masking at a Massachusetts health system led to a decrease in COVID-19 cases among healthcare workers, even as the virus spread increased in the surrounding community, according to a study published Oct. 21 by Occupational Medicine.

The study — led by researchers at Cambridge (Mass.) Health Alliance, a Harvard-affiliated community health system, and the Harvard University T.H. Chan School of Public Health — compared COVID-19 infection rates between Cambridge Health Alliance and Massachusetts residents. The study period was March 17, when the first health system employee tested positive, to May 6, when Massachusetts began requiring masking in public places where social distancing isn’t possible. 

From March 17 to April 1 (five days after Cambridge Health Alliance implemented universal masking), COVID-19 infections among healthcare workers at the health system were increasing at the same, relative steep rate as that of the surrounding community. After the masking intervention April 1-20, Cambridge Health Alliance flattened and decreased its curve significantly, while community infections continued to rise until Massachusetts reached its COVID-19 incidence peak on April 20, researchers said. Both infection rates declined at similar rates after the surge peaked in Massachusetts April 20.

“We found clear benefits to universal masking for preventing infectious spread within the work environment,” study senior author Stefanos Kales, MD, division chief of occupational and environmental medicine at Cambridge Health Alliance and a professor at both Harvard Medical School and Harvard T.H. Chan School of Public Health, concluded. “Our findings suggest that a universal masking policy should be implemented and maintained in healthcare settings as well as within indoor businesses when physical distancing and ventilation may be inadequate.”

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