A vital new study shows how well masks work—and why we need better ones

Throughout the COVID-19 pandemic, guidance on mask-wearing has varied tremendously across, and within, countries—as have the methods used to encourage mask use and enforce mask mandates. If authorities in places that have shunned masks are claiming that’s due to a lack of substantial evidence of their effectiveness in mitigating the community spread of COVID-19, a first-of-its-kind study now fills that gap in data.

The study is the first designed to assess masking’s impact, not on preventing the virus from entering your airways, as others have, but in reducing overall infection rates among a population cluster. The work, which also explored the most effective ways of promoting mask-wearing, found that symptomatic infections declined by at least 9.3% when masks were worn by just a small portion of the community. The researchers performed the study precisely to help inform public health policy, and hope that can now happen just as masks have become critical again.

Researchers randomly selected 600 villages within the same upazilas, Bangladeshi administrative regions. In half, they distributed masks to the population via specific promotional techniques, including free distribution in public places like mosques and marketplaces; communication from trusted leaders like the prime minister and a national cricketing hero; and reinforcement by imams at Friday prayers. The remaining 300 villages, the control group, went about as normal: They weren’t discouraged from using masks, but researchers didn’t intervene. In those groups, only 13% of people consistently wore masks; in the targeted groups, 42% did.

Most other mask studies have been performed in healthcare settings or in the lab; this one took place in a country with the eighth highest population, and a high population density of 1,265 people per square kilometer. “It’s certainly a society where the possibility of intense transmission is very likely,” says coauthor Ashley Styczynski, an infectious disease fellow at Stanford University. Bangladesh also had a high initial uptake of masks, which dipped soon after, even as cases were rising. “So, we felt that there was a lot of room for improving mask use.”

At five and nine weeks of the eight-week study, they surveyed people about symptoms. Of those who reported symptoms, researchers took blood, if they consented, to test for antibodies. They found that the 29% increase in mask usage between the villages that were given masks and the control villages led to a 9.3% decrease in transmissions. In addition (and perhaps more importantly), they found surgical masks provided more protection, with 12% fewer infections, and an even higher infection-rate drop among older people—a crucial finding for a group that faces the biggest risk of morbidity and mortality.

The team has been in touch with science officers at the World Health Organization (WHO) since the beginning of the study, “because we wanted this to be policy-relevant in the end,” Styczynski says. They’ve also passed the research on to the CDC. They hope the new data will bolster the attempts of governments aiming to encourage mask-wearing. And, authorities looking to administer mandates effectively, without using draconian measures, can look to the promotional techniques from the study for clues. The researchers have started advising local policymakers in Bangladesh, India, Nepal, and Pakistan about implementation. Employing cricket stars and imams are, of course, not directly duplicable in other countries—”the U.S. is not the same as rural Bangladesh,” Styczynski says—but, the fundamentals are still relevant.

In the U.S., “I don’t think that 100% of citizens will suddenly be convinced that they want to wear masks,” Styczynski admits but notes there were people who said they were holding out on mask usage, citing the need more evidence. The practice has become even more critical with the spread of delta, since the original 70% herd immunity target is no longer high enough—especially as a large population remains unvaccinated, including all schoolchildren. As we trend toward more indoor activities as the weather gets colder, “I think we’ll see those transmissions continue at a fairly substantial rate,” she says. “This is a real opportunity to make these transitions safer.”

Even more promising is that the 9.3% figure is probably a low estimate. The study rolled out between virus surges, and before the delta variant, which is much more transmissible. Only about a third of those reporting symptoms consented to blood tests, and they didn’t measure asymptomatic or even mildly symptomatic cases. And in Bangladesh, like many rural developing nations, much of daily life takes place outdoors, compared to higher-income countries, where there’s likely to be more indoor spread.

Most importantly, the figure was from an increase in mask usage of just 29%; if authorities can get that number even higher, they’re likely to see even more protection. It’s not an easy feat for governments to get mask usage up, especially after high-profile relaxation of the guidelines as the CDC announced in the spring, but Styczynski is optimistic. “I think it’s absolutely not too late for the U.S.”