Three new studies—including two conducted by CDC that examined mask mandates in schools and a separate, randomized controlled trial of mask-wearing in Bangladesh—provide new evidence that masks can help prevent the spread of Covid-19.

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How effective are school mask mandates?

For one of the CDC studies, researchers looked at 999 schools in Maricopa and Pima counties in Arizona, where schools started in July. Of those schools, 21% had a mask requirement at the start of the school year, around 30% added a mandate later, and 48% did not mandate masks.

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The researchers defined a Covid-19 outbreak as an instance in which a school had at least two confirmed Covid-19 cases among students or staff within a two-week period at least a week after the start of the school year.

Out of the 191 schools that experienced an outbreak between July 15 and August 31, 59.2% did not have a mask mandate in place. In comparison, 8.4% of the schools that had a mask mandate in place at the start of the school year experienced an outbreak, the researchers found.

A second CDC study examined 520 counties throughout the United States and found that counties where school mask mandates were in effect saw smaller increases of pediatric Covid-19 case rates than other counties.

Specifically, from one to seven days before the start of school to seven to 13 days after the start of school, counties with school mask mandates saw an average increase of 16.32 Covid-19 cases per 100,000 children. Other counties, meanwhile, experienced an average increase of 34.85 cases per 100,000 children.

CDC cautioned that, because the study examined only 16.5% of U.S. counties, it may not apply to the entire country.

How effective are masks in general?

Separately, researchers for another study conducted a randomized controlled trial among 340,000 adults in 600 villages in Bangladesh to evaluate the effectiveness of masks in preventing coronavirus transmission.

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The study, which is still undergoing peer review, was run by researchers from Yale University, the University of California, Berkeley School of Public Health, and Stanford University.

The researchers distributed masks to people at their homes and in crowded public places, and they involved community and religious leaders in providing information on why mask-wearing was important. They also encouraged village residents to ask anyone not wearing a mask to wear one.

The researchers found that, among villagers randomly assigned to receive the intervention, mask-wearing increased around 30 percentage points, driving a 9% reduction in Covid-19 cases. The quality of masks used mattered, too: In communities where surgical masks rather than cloth masks were promoted, Covid-19 cases fell by 11%.

The benefits were especially significant in people over the age of 50, the researchers found, with Covid-19 rates dropping by 23% among those ages 50 to 60 and by 35% among those over the age of 60. Writing in the New York Times, the researchers said the study "does not suggest that only older people need to wear masks, but rather that widespread community mask wearing reduces Covid-19 risk, especially for older people."

The researchers also tested the filtration provided by different kinds of masks. They found that surgical masks were more effective than cloth masks at preventing virus particles from passing through them—even after the surgical masks had been worn, bunched up in pockets and purses, and washed with soap and water up to 10 times.

Masks with better filtration and fit than surgical masks, such KF94 or KN95 masks, may provide even better filtration, the researchers suggested. (Coleman, The Hill, 9/24; Rabin, New York Times, 9/24; Abaluck et al., New York Times, 9/26)