Surgical masks reduce COVID-19 spread, large-scale study shows – Stanford Medical Center Report

Researchers found that surgical masks impede the spread of COVID-19 and that just a few, low-cost interventions increase mask-wearing compliance.

Sep 1 2021
Providing a woman with a surgical mask at a market in Bangladesh

Providing free masks to people in rural Bangladesh was one measure researchers tested to limit the spread of COVID-19.     
Innovations for Poverty Action

A large, randomized trial led by researchers at Stanford Medicine and Yale University has found that wearing a surgical face mask over the mouth and nose is an effective way to reduce the occurrence of COVID-19 in community settings.

It also showed that relatively low-cost, targeted interventions to promote mask-wearing can significantly increase the use of face coverings in rural, low-income countries. Based on the results, the interventional model is being scaled up to reach tens of millions of people in Southeast Asia and Latin America over the next few months.

The findings were released Sept. 1 on the Innovations for Poverty Action website, prior to their publication in a scientific journal, because the information is considered of pressing importance for public health as the pandemic worsens in many parts of the world.

“We now have evidence from a randomized, controlled trial that mask promotion increases the use of face coverings and prevents the spread of COVID-19,” said Stephen Luby, MD, professor of medicine at Stanford. “This is the gold standard for evaluating public health interventions. Importantly, this approach was designed be scalable in lower- and middle-income countries struggling to get or distribute vaccines against the virus.”

Luby shares senior authorship with Ahmed Mushfiq Mobarak, PhD, professor of economics at Yale, of a paper describing the research. The lead authors are Ashley Styczynski, MD, MPH, an infectious disease fellow at Stanford; Jason Abaluck, PhD, professor of economics at Yale; and Laura Kwong, PhD, a former postdoctoral scholar at Stanford who is now an assistant professor of environmental health sciences at the University of California-Berkeley.

The researchers also partnered with Innovations for Poverty Action, a global research and policy nonprofit organization.

Increasing mask use in rural Bangladesh

The researchers enrolled nearly 350,000 people from 600 villages in rural Bangladesh. Those living in villages randomly assigned to a series of interventions promoting the use of surgical masks were about 11% less likely than those living in control villages to develop COVID-19, which is caused by infection with the SARS-CoV-2 virus, during the eight-week study period. The protective effect increased to nearly 35% for people over 60 years old.

Providing free masks, informing people about the importance of covering both the mouth and nose, reminding people in-person when they were unmasked in public, and role-modeling by community leaders tripled regular mask usage compared with control villages that received no interventions, the researchers found.

In the intervention villages, they also saw a slight increase in physical distancing in public spaces, such as marketplaces. This finding indicates that mask-wearing doesn’t give a false sense of security that leads to risk-taking behaviors — a concern cited by the World Health Organization during the early days of the pandemic when its officials were considering whether to recommend universal masking.

“Our study is the first randomized controlled trial exploring whether facial masking prevents COVID-19 transmission at the community level,” Styczynski said. “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.”

Cloth vs. surgical masks

There were significantly fewer COVID-19 cases in villages with surgical masks compared with the control villages. (Although there were also fewer COVID-19 cases in villages with cloth masks as compared to control villages, the difference was not statistically significant.) This aligns with lab tests showing that surgical masks have better filtration than cloth masks. However, cloth masks did reduce the overall likelihood of experiencing symptoms of respiratory illness during the study period.

Bangladesh is a densely populated country in South Asia. It was chosen as the site of the trial for several reasons: One, mask promotion is considered vital in countries where physical distancing can be difficult; two, Innovations for Poverty Action Bangladesh had already established a research framework in the country; and three, many local partners were eager to support a randomized, controlled trial of masking.

“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,” Kwong said. “So we collaborated with behavioral scientists, economists, public health experts and religious figures to design ways to promote mask use at a community level.”

Despite a growing body of scientific evidence that masks reduce the spread of the virus that causes COVID-19, it has been difficult to increase mask-wearing, particularly in low-resource countries and among people living in remote or rural areas. In June of 2020, only one-fifth of Bangladeshis in public areas were wearing a mask that properly covered the mouth and nose despite a nationwide mask mandate that was in effect at the time.

Educational and behavioral interventions

The researchers wanted to explore whether it was possible to increase mask-wearing in Bangladeshi villages through a variety of educational and behavioral interventions over an eight-week study period: Free cloth or washable, reusable surgical masks were given to people at home and in marketplaces, mosques and other public spaces; notable Bangladeshi figures, including the prime minister, a star cricket player and a leading imam, provided information about why wearing a mask is important; people appearing in public places without masks were reminded to wear masks; and community leaders modeled mask-wearing.

The villages were selected by researchers at Innovations for Poverty Action Bangladesh. The researchers paired 600 villages countrywide based on population size and density, geographic location, and any available COVID-19 case data. For each of the 300 pairs of villages, one was randomly assigned to receive the interventions while the other served as a control and received no interventions. Two-thirds of the intervention villages received surgical masks, while the other one-third received cloth masks. In total, 178,288 people were in the intervention group, and 163,838 people were in the control group.

The interventions were rolled out in waves from mid-November to early January. For eight weeks after the interventions, observers stationed at various public places in both the control and intervention villages recorded whether a person was wearing a mask over both their mouth and nose and whether they appeared to be practicing physical distancing — that is, staying at least an arm’s length away from all other people.

At week 5 and week 9, villagers were asked if they had experienced any COVID-19 symptoms — including fever, cough, nasal congestion and sore throat — during the previous month and, if so, whether they would provide a blood sample to test for the presence of SARS-CoV-2. About 40% of symptomatic people consented to subsequent blood collection.

We saw the largest impact on older people who are at greater risk of death from COVID-19.

The observers found that just over 13% of people in the villages that received no interventions wore a mask properly, compared with more than 42% of people in the villages where each household received free masks and in-person reminders to wear them. Physical distancing was observed 24.1% of the time in control villages and 29.2% of the time in intervention villages.

About 7.6% of people in the intervention villages reported COVID-19 symptoms compared with about 8.6% of those in the control villages during the eight-week study period — a statistically significant difference that indicates a roughly 12% reduction in the risk of experiencing respiratory symptoms.

The researchers found that among the more than 350,000 people studied, the rate of people who reported symptoms of COVID-19, consented to blood collection and tested positive for the virus was 0.76% in the control villages and 0.68% in the intervention villages, showing an overall reduction in risk for symptomatic, confirmed infection of 9.3% in the intervention villages regardless of mask type.

When the researchers considered only those villages that received surgical masks (omitting villages that received cloth masks), the reduction in risk increased to 11%. Furthermore, the protective effect of surgical masks was greater for older people: As a group, those ages 50 to 60 were 23% less likely to develop COVID-19 if they wore a surgical mask, and those over 60 were 35% less likely if they did.

“This is statistically significant and, we believe, probably a low estimate of the effectiveness of surgical masks in community settings,” Styczynski said. The fact that the study was conducted at a time when the rate of transmission of COVID-19 in Bangladesh was relatively low, that a minority of symptomatic people consented to blood collection to confirm their disease status, and that fewer than half of the people in the intervention villages used facial coverings means the true impact of near-universal masking could be much more significant — particularly in areas with more indoor gatherings and events, she noted.

“If mask-wearing rates were higher, we would expect to see an even bigger impact on transmission,” Luby said. “But even at this level, we saw the largest impact on older people who are at greater risk of death from COVID-19.”

The interventions are now being rolled out in other parts of Bangladesh and in Pakistan, India, Nepal and parts of Latin America. But the researchers also hope there are lessons in the study for Americans.

“Unfortunately, much of the conversation around masking in the United States is not evidence-based,” Luby said. “Our study provides strong evidence that mask wearing can interrupt the transmission of SARS-CoV-2. It also suggests that filtration efficiency is important. This includes the fit of the mask as well as the materials from which it is made. A cloth mask is certainly better than nothing. But now might be a good time to consider upgrading to a surgical mask.”

The study was supported by a grant from GiveWell.org to Innovations for Poverty Action.

Researchers from Innovation for Poverty Action; the University of California-Berkeley; Johns Hopkins Bloomberg School of Public Health; the NGRI North South University in Dhaka, Bangladesh; and Deakin University in Melbourne also contributed to the study.


Stanford Medicine integrates research, medical education and health care at its three institutions – Stanford University School of Medicine, Stanford Health Care (formerly Stanford Hospital & Clinics), and Lucile Packard Children’s Hospital Stanford. For more information, please visit the Office of Communication & Public Affairs site at http://mednews.stanford.edu.

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