A study involving more than 340,000 people in Bangladesh offers some of the strongest real-world evidence yet that mask use can help communities slow the spread of COVID-19.
The research, conducted across 600 villages in rural Bangladesh, is the largest randomized trial to demonstrate the effectiveness of surgical masks, in particular, to curb transmission of the coronavirus. Though previous, smaller studies in laboratories and hospitals have shown that masks can help prevent the spread of COVID-19, the new findings demonstrate that efficacy in the real world — and on an enormous scale.
"This is really solid data that combines the control of a lab study with real-life actions of people in the world to see if we can get people to wear masks, and if the masks work," said Laura Kwong, an assistant professor of environmental health sciences at the University of California, Berkeley, and one of the co-authors of the study.
The preprint study was posted online Wednesday by the nonprofit organization Innovations for Poverty Action and is currently undergoing peer review.
The findings have important implications for countries that are relying on mitigation measures to slow the virus's spread until vaccines are more readily available. But there are also applicable lessons for nations like the United States, where some communities are reimposing mask mandates to stem outbreaks of the delta variant.
"The policy question we were trying to answer was: If you can distribute masks and get people to wear them, do they work?" said study co-author Mushfiq Mobarak, a professor of economics at Yale University.
For five months beginning last November, Mobarak and his colleagues tracked 342,126 adult Bangladeshis and randomly selected villages to roll out programs to promote their usage, which included distributing free masks to households, providing information about their importance and reinforcing their use in the community.
Among the roughly 178,000 individuals who were encouraged to wear them, the scientists found that mask-wearing increased by almost 30% and that the change in behavior persisted for 10 weeks or more. After the program was instituted, the researchers reported an 11.9% decrease in symptomatic COVID-19 symptoms and a 9.3% reduction in symptomatic seroprevalence, which indicates that the virus was detected in blood tests.
While the effect may seem small, the results offer a glimpse of just how much masks matter, Mobarak said.
"A 30% increase in mask-wearing led to a 10% drop in COVID, so imagine if there was a 100% increase — if everybody wore a mask and we saw a 100% change," he said.
The scientists said masks significantly reduced symptomatic infections among older adults, and found that surgical masks were more effective than cloth versions.
Kwong said those findings may be especially important for countries such as the U.S., where people spend much more time indoors compared to those in rural Bangladesh.
"Right now, places say to cover your face but they don't say what type of face covering," she said. "If schools and workplaces and other indoor public spaces are going to mandate masks, they should be working to mandate surgical masks."
Ajay Sethi, an epidemiologist and associate professor of population health sciences at the University of Wisconsin-Madison, who was not involved with the study, called the research "thoughtfully put together" and "impressive on many different levels."
He added that the project demonstrated that strategies can be effectively implemented in communities to change mask behavior. In the U.S., public health officials have struggled to promote their usage after masks were politicized early in the pandemic.
"Normative behavior is what needs to be targeted," Sethi said. "It’s not just mask use that needs to be adopted, it’s also an understanding of why masks need to be used and reinforcement that the virus is serious."
Kwong and her colleagues are now expanding their research to include other villages and cities in Asia and sub-Saharan Africa. The researchers also intend to track the effect of masks on asymptomatic transmission.
Dr. Isaac Bogoch, an infectious diseases physician and associate professor of medicine at the University of Toronto, said the research helps reinforce what was known about the effectiveness of masks, but he stressed that people should see them as just one of multiple interventions needed to stop the spread of Covid.
"We need vaccinations, better ventilation in indoor settings, crowd control, physical distancing — all these different added layers of protection," he said. "Masks certainly help, but we can't mask our way out of the pandemic."
This story first appeared on NBCNews.com.