The study also found clear evidence that surgical masks are better at reducing the spread of symptomatic COVID-19 than cloth masks. In focus groups, Bangladeshi participants said they preferred cloth masks because they seemed to be more durable. But the researchers found that, on the one hand, surgical masks were more efficient, even after being washed 10 times with soap and water. “On the other hand, we found only mixed evidence about cloth masks,” Jason Abaluck, a co-author of the study and a professor at Yale, told me. People wearing cloth masks had fewer symptoms, such as coughs, than the control group, which suggests some effect. But cloth-mask wearers didn’t have significantly fewer coronavirus antibodies as determined by blood tests. “We cannot reject that [cloth masks] have zero or only a small impact on symptomatic SARS-CoV-2 infections,” Abaluck wrote along with Mushfiq Mobarak of Yale, Laura Kwong of UC Berkeley, Stephen Luby and Ashley Styczynski of Stanford, and other researchers.

Creating a social norm is hard work. The pro-masking intervention in this study was aggressive, extensive, and expensive. In all, the researchers distributed more than 1 million masks. Free distribution of masks was important. But of all the interventions, mask promotion—that is, paying individuals to remind people on the street to cover their face—seemed to have the biggest effect. “Reminders from people in the village almost acted as booster shots for masking,” Abaluck told me. The research team is currently working on scaling up its intervention in countries including Pakistan, India, and Nepal.