The study, led by researchers at Stanford Medicine and Yale University, was a large-scale, randomized trial using 350,000 people from 600 villages in rural Bangladesh. Those living in villages were randomly assigned a number of interventions to promote the use of masks.
Researchers found that those living in the villages with such interventions were around 11% less likely to develop COVID-19 than those who did not. The effectiveness increased to nearly 35% for people over 60 years old, according to Stanford Medicine.
“Our study is the first randomized controlled trial exploring whether facial masking prevents COVID-19 transmission at the community level,” said Ashley Styczynski, one of the lead authors and an infectious disease fellow at Stanford. “It’s notable that even though fewer than 50% of the people in the intervention villages wore masks in public places, we still saw a significant risk reduction in symptomatic COVID-19 in these communities, particularly in elderly, more vulnerable people.”
The study also looked at the effect of using cloth instead of surgical masks: Cloth masks did reduce the overall likelihood of experiencing symptoms of respiratory illness, but it was not as effective as using a surgical mask.
“We saw an opportunity to better understand the effect of masks, which can be a very important way for people in low-resource areas to protect themselves while they wait for vaccines,” said Laura Kwong, a former postdoctoral scholar at Stanford and assistant professor at the University of California-Berkeley. “So we collaborated with behavioral scientists, economists, public health experts and religious figures to design ways to promote mask use at a community level.”
The study paired the 600 villages, assigning one with mask distribution and some promotion of policy, which involved notable Bangladeshi figures – such as the prime minister, a star cricket player and an imam – wearing masks and explaining why wearing masks was important. Additionally, individuals not wearing masks nor social distancing were regularly reminded to do so.
In villages without these interventions, around 13% of people wore masks properly compared to 42% in villages with the guidance. Social distancing only slightly improved with 24.1% in the control villages versus 29.2% in the intervention villages.
“This is statistically significant and, we believe, probably a low estimate of the effectiveness of surgical masks in community settings,” Styczynski said.