Finally, A Randomized Trial of Mask Wearing

By Paul Shafer, PhD
September 10, 2021

Paul Shafer is an assistant professor in the Department of Health Law, Policy and Management at the Boston University School of Public Health. His research focuses on the effects of state and federal health insurance policy on coverage, health care use and health equity.

As COVID-19 cases surge once again and vaccination rates remain stubbornly low, policymakers are returning to other measures to stop the spread of this relentless virus. In late July, the Centers for Disease Control and Prevention recommended indoor masking for fully vaccinated people in areas with high community spread and universal masking in all educational settings. This led many states and local governments to reinstitute mask mandates, creating inflamed battles at school board and city council meetings.

The body of evidence underlying these policy decisions has consisted largely of observational studies, which often lack the strength of randomized trials because those who choose to wear masks on their own or states that adopt stricter measures are likely fundamentally different than those who do not. A new study published in an Innovations for Poverty Action working paper gets around this by using a randomized intervention designed to increase mask wearing, allowing us to see what happens when more people do.

A large team of international collaborators conducted this randomized trial in rural Bangladesh from November 2020 to April 2021, covering 600 villages with over 340,000 adults. The intervention, known as the NORM model, included 1) free masks (1/3 cloth, 2/3 surgical), 2) instructions on how to wear them, 3) in-community mask promoters and 4) endorsement by religious leaders for randomly selected villages. The control villages received none of those things, though they were not prevented from wearing masks on their own.

The researchers found the intervention tripled the level of mask wearing and, in turn, led to decreases in both self-reported COVID symptoms (-11.9%) and symptomatic seroprevalence, or the presence of SARS-CoV-2 antibodies in the blood of symptomatic people (-9.3%), in the treatment villages relative to the control villages. The effects of the intervention held over a 10-week period, but waned in the months after the study ended as mask wearing declined.

The study also weighed in on two other hotly debated questions about masks. First, the intervention increased physical distancing by 5 percentage points (24.1% versus 29.2%), countering concerns that increased mask wearing may lead to more risky social behavior. Second, the study provided suggestive evidence that surgical masks may be more effective in the real world at reducing COVID-19 spread than cloth masks, building upon prior lab-based studies

Symptomatic seroprevalence decreased by 11.2% in villages randomized to wear surgical masks, while cloth masks produced no statistically significant reduction. However, cloth masks were provided to about half as many people, raising questions about statistical power and leading one of the researchers to note, “We are very careful not to say surgical [is better than] cloth … but instead to say, we find clear evidence that surgical works.

So, how should we think about translating this evidence to the U.S.? Study co-author Jason Abaluck extrapolated their results on Twitter in terms I found helpful. Given current COVID death rates in the U.S., for every 600 people who mask in public for a year, we can prevent one death. We would need to take 10,000 drivers off the road to achieve the same benefit.

This study adds great heft to the body of evidence on the effectiveness of masking. But, with its focus on adults, it does not directly address the debate over masks in schools, where children under 12 cannot yet be vaccinated, that continues to rage. We cannot improve school ventilation or community vaccination rates overnight, but local policymakers can require masks in schools, an idea I already believed was a no-brainer and is only strengthened by this study. As epidemiologist Whitney Robinson put it, masks are “better than the nothing that’s on the table in many districts.”

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