BOSTON (CBS) — A new study suggests that masks are highly effective at preventing coronavirus infections. Researchers said that after all 78,000 staff members at Mass General Brigham hospitals were required to wear surgical masks in March “the rate of COVID-19 infection among health care workers dropped significantly.”

The study found that before there was a universal mask policy, the coronavirus positivity rate climbed exponentially from 0% to 21%, and cases were doubling every 3.6 days. After the policy was put in place, the positivity rate declined to 11% even as Massachusetts case numbers increased.

“These results support universal masking as part of a multipronged, infection-reduction strategy in health care settings,” said study author Dr. Deepak L. Bhatt of Brigham and Women’s Hospital in a statement. “While we studied health care workers, the results also apply to other situations in which social distancing is not possible. For those who have been waiting for data before adopting the practice, this paper makes it clear: Masks work.”

The study notes that other actions taken by hospitals may have also contributed to driving down case numbers. But researchers said they see clear evidence that masks made a big difference.

“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, of Mass General Brigham, in a 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.”

The CDC reviewed the Mass General Brigham study and while that research only looked at surgical masks, the agency says there is “increasing evidence that cloth face coverings help prevent people who have COVID-19 from spreading the virus to others.”