After Brigham and Women's Hospital in Boston began requiring that nearly everyone in the hospital wear masks, new coronavirus infections diagnosed in its staffers dropped by half — or more.

Brigham and Women’s epidemiologist Dr. Michael Klompas said the hospital mandated masks for all health care staffers on March 25, and extended the requirement to patients as well on April 6.

"When we first began our universal masking policy, we had 12 to 14 new infections per day among our health care workers," he said. "And then after we instituted employee masking, that number dropped down to around eight."

It dropped still further to about six new infections a day once patients had to wear masks, too.

This is by no means gold-plated evidence, Klompas said, and correlation is not causation, but it does suggest that wearing simple masks may help stem the spread of the virus, whether in a hospital or out in public.

He hopes to find out whether other hospitals in the Partners Healthcare system, which imposed the "universal mask" policy system-wide, have seen similar drops. "If it's a consistent pattern, I'd be more apt to believe it," he said.

It's thought that employees most often catch the virus outside the hospital, Klompas said, so that could explain why the number of new infections did not drop all the way to zero.