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Boston hospital worker study shows masks are effective in preventing spread of coronavirus

Ronald Zak/Associated Press

As COVID-19 infections soared in Massachusetts during the height of the outbreak, the infection rate among employees at Mass General Brigham hospitals dropped significantly once all workers were required to wear masks, suggesting face coverings do work in preventing the spread of the deadly contagion, according to a new study.

The study, led by investigators from Brigham and Women’s Hospital, which is part of the Mass General Brigham network, was published online Tuesday in the medical journal JAMA.

“While we studied health care workers, the results also apply to other situations in which social distancing is not possible,” said Dr. Deepak L. Bhatt, head of interventional cardiovascular programs at Brigham and Women’s, in a statement. “For those who have been waiting for data before adopting the practice, this paper makes it clear: Masks work.”


According to the statement, the study looked at infection rates among workers at Mass General Brigham’s 12 hospitals before and after the mask policy took effect in late March.

Bhatt and his team analyzed data from March 1 to April 30, and of 9,850 workers tested during that period, the infection rate was 13 percent, the statement said.

Researchers compared data during the “pre-intervention period” before the mask policy took effect, with the later period once face coverings were required, according to the statement from Brigham and Women’s.

Before the mask policy was implemented, the statement said, the infection rate among employees spiked “exponentially” from 0 percent to 21 percent, with cases doubling every 3.6 days.

Once masks were required, the positive test rate “decreased linearly from 15 percent to 11 percent,” the statement said.

The statewide infection rate peaked in mid-April at about 29 percent, according to public health officials.

“This is the most direct COVID-19 research data to this point that is based on testing of health care workers pre- and post-implementation of universal masking policies,” said Dr. Dean Hashimoto, chief medical officer for Occupational Health Services at Mass General Brigham, in the statement. “When our Infection Control leaders announced a universal masking policy early in the pandemic it was a bold move, especially at a time when, like all health systems, we were facing PPE shortages. But the results of this study demonstrate that requiring masks for all hospital staff regardless of role in the organization was critical to protecting our employees.”


Whether or not to wear masks has emerged as one of the most politically charged debates surrounding the pandemic, which has killed more than 8,100 Massachusetts residents and more than 135,000 people nationwide.

President Trump wore a mask for the first time Saturday during a visit to a military hospital, though the CDC months earlier recommended Americans cover their faces when leaving home, especially if they plan to be around other people.

People close to Trump have told The Associated Press that he feared a mask would make him look weak and shift focus to the public health crisis rather than the economic recovery.

On Tuesday, CDC director Dr. Robert Redfield sounded a bullish note on masks during a briefing, telling reporters “the data is really clear — they work,” CNN reported.

“We’re not defenseless against this virus,” Redfield said, according to the network. “We actually have face coverings and I do think the more confidence that the American public has — that face coverings are not a symbol, but they’re actually a very important preventive intervention that can really block this virus.”


Material from the Associated Press and prior Globe stories was used in this report.

Travis Andersen can be reached at travis.andersen@globe.com.