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A large, real-world test of face masks in Bangladesh shows that masks work to reduce community spread of COVID-19. It also shows that surgical masks are more effective than cloth face coverings.

The study, which was published ahead of peer review, demonstrates the power of careful investigation and offers a host of lessons about mask wearing that will be important worldwide. One key finding of the study, for example, is that wearing a mask doesn’t lead people to abandon social distancing, something public health officials had feared might happen if masks gave people a false sense of security.

“What we really were able to achieve is to demonstrate that masks are effective against COVID-19, even under a rigorous and systematic evaluation that was done in the throes of the pandemic,” said Ashley Styczynski, MD, who was an infectious disease fellow at Stanford University when she collaborated on the study with other colleagues at Stanford, Yale, and Innovations for Poverty Action (IPA), a large research and policy nonprofit organization that currently works in 22 countries.

“And so, I think people who have been holding out on wearing masks because [they] felt like there wasn’t enough evidence for it, we’re hoping this will really help bridge that gap for them,” she said.

It included more than 600 unions — or local governmental districts in Bangladesh — and roughly 340,000 people.

Half of the districts were given cloth or surgical face masks along with continual reminders to wear them properly; the other half were tracked with no intervention. Blood tests of people who developed symptoms during the study verified their infections.

Compared to villages that didn’t mask, those in which masks of any type were worn had about 9% fewer symptomatic cases of COVID-19. The finding was statistically significant and was unlikely to have occurred by chance alone.

“Somebody could read this study and say, ‘OK, you reduced COVID-19 by 9%. Big deal.’ And what I would respond to that would be that if anything, we think that that is a substantial underestimate,” Styczynski said.

One reason they think they underestimated the effectiveness of masks is that they only tested people who were having symptoms, so people who had only very mild or asymptomatic infections were missed.

Another reason is that among people who had symptoms, only one third agreed to undergo a blood test. The effect may have been bigger had participation been universal.

Local transmission may have played a role, too. Rates of COVID-19 in Bangladesh were relatively low during the study. Most infections were caused by the B.1.1.7, or Alpha, variant.

Since then, Delta has taken over. Delta is thought to be more transmissible, and some studies have suggested that people infected with Delta shed more viral particles. Masks may be more effective when more virus is circulating.

The investigators also found important differences by age and by the type of mask. Villages in which surgical masks were worn had 11% fewer COVID-19 cases than villages in which masks were not worn. In villages in which cloth masks were worn, on the other hand, infections were reduced by only 5%.

The cloth masks were substantial. Each had three layers ― two layers of fabric with an outer layer of polypropylene. On testing, the filtration efficiency of the cloth masks was only about 37%, compared to 95% for the three-layer surgical masks, which were also made of polypropylene.

Masks were most effective for older individuals. People aged 50 to 60 years who wore surgical masks were 23% less likely to test positive for COVID compared to their peers who didn’t were masks. For people older than 60, the reduction in risk was greater — 35%.

Rigorous Research

The study took place over a period of 8 weeks in each district. The interventions were rolled out in waves, with the first starting in November 2020 and the last in January 2021.

Investigators gave each household free cloth or surgical face masks and showed families a video about proper mask wearing with promotional messages from the prime minister, a head imam, and a national cricket star. They also handed out free masks.

Previous studies have shown that people aren’t always truthful about wearing masks in public. In Kenya, for example, 88% of people answering a phone survey said that they wore masks regularly, but researchers determined that only 10% of them actually did so.

Investigators in the Bangladesh study didn’t just ask people if they’d worn masks, they stationed themselves in public markets, mosques, tea stalls, and on roads that were the main entrances to the villages and took notes.

They also tested various ways to educate people and to remind them to wear masks. They found that four factors were effective at promoting the wearing of masks, and they gave them an acronym ― NORM.

• N, for no-cost masks;

• O, for offering information through the video and local leaders;

• R, for regular reminders to people by investigators who stand in public markets and offer masks or encourage anyone who wasn’t wearing one or wearing it correctly;

• M, for modeling, in which local leaders, such as imams, wear masks and remind their followers to wear them.

These four measures tripled the wearing of masks in the intervention communities, from a baseline level of 13% to 42%. People continued to wear their masks properly for about 2 weeks after the study ended, indicating that they’d gotten used to wearing them.

Styczynski said that nothing else ― neither text message reminders, nor signs posted in public places, nor local incentives ― moved the needle on mask wearing.

Saved Lives and Money

The study found that the strategy was cost-effective, too. Giving masks to a large population and getting people to use them costs about $10,000 per life saved from COVID, on par with the cost of deploying mosquito nets to save people from malaria, Styczynski said.

“I think that what we’ve been able to show is that this is a really important tool to be used globally, especially as countries have delays in getting access to vaccines and rolling them out,” she said.

Styczynski said masks will continue to be important even in countries such as the United States, where vaccines aren’t stopping transmission 100% and there are still large portions of the population who are unvaccinated, such as children.

“If we want to reduce COVID-19 here, it’s really important that we consider the ongoing utility of masks, in addition to vaccines, and not really thinking of them as one or the other,” she said.

The study was funded by a grant from GiveWell.org. The funder had no role in the study design, interpretation, or the decision to publish.

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