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Surgical Masks Indisputably Reduce COVID-19 Spread, "Gold Standard" Study Finds


Maddy Chapman

Maddy is a Editor and Writer at IFLScience, with a degree in biochemistry from the University of York.

Editor & Writer

surgical face masks

The gold standard study also found that surgical masks are more effective than cloth ones. Image credit: Maridav/

The wearing of face masks to prevent the spread of COVID-19 has been a polarizing topic since they were mandated in many countries following the start of the pandemic. There has been lots of previous research that has shown their efficacy, but a huge, new trial in Bangladesh has shown categorically, scientists say, that surgical masks do reduce the spread of SARS-CoV-2 – the virus that causes COVID-19.

The randomized trial – the gold standard of clinical trials – provides real-world evidence that should “end any scientific debate about whether masks can be effective in combating covid at the population level,” Jason Abaluck, an economist at Yale who helped lead the study, told The Washington Post


The study was posted as a preprint to the Innovations for Poverty Action nonprofit website is currently undergoing peer-review at the journal Science, according to the Post. 

The trial, which took place across 600 villages in Bangladesh between November 2020 and April 2021, involved over 342,000 adults, making it the largest randomized study on mask-wearing and COVID spread to date, the Post reports. 

Of these participants, 178,000 were in the intervention group who were encouraged to wear masks, whilst the remaining 163,000 or so acted as a control group and received no “intervention”. Mask wearing more than tripled after the intervention, with an increase of 28.8 percent between the control and intervention groups. The researchers estimate an 11.9 percent reduction in COVID symptoms in the mask-wearing group and a 9.3 percent reduction in symptomatic infections.

“Our results should not be taken to imply that masks can prevent only 10% of COVID-19 cases, let alone 10% of COVID-19 mortality,” the authors write in the study. They report that while mask-wearing did increase after the intervention, it was not universal – only 29 more people out of every 100 chose to wear masks. “The total impact with near-universal masking – perhaps achievable with alternative strategies or stricter enforcement – may be several times larger than our 10% estimate.”


To come to these conclusions, the authors implemented a four-point mask promotion strategy. Everyone in the intervention group was delivered free masks, provided with information on the importance of masking, given regular in-person reminders, and witnessed respected community leaders publicly endorsing masking. 

The level of mask-wearing was observed by plain-clothed staff in mosques, markets, main entrance roads to villages, and tea stalls. After five to nine weeks, participants were surveyed for symptoms and at 10 to 12 weeks blood samples were taken to check for SARS-CoV-2 antibodies and determine the number of infections.

As we previously reported, not all masks are created equal, and this new study found no different. In groups given surgical masks, symptomatic infections were 11.2 percent lower than in the control group. In the over-60s this reduction was even more dramatic: 34.7 percent. Cloth masks, on the other hand, had no effect on the number of infections. They did, however, reduce COVID symptoms, although the effect was much less – 30 to 80 percent – than in surgical mask villages. Abaluck emphasized to the Post that these findings are by no means evidence that cloth masks are ineffective.

As for the longevity of the intervention, after five months “the impact … faded,” with fewer people maintaining regular mask-wearing. But 10 percent more people in the intervention group wore masks compared to those in the control, suggesting the team’s intervention strategy was effective, and with continued use could successfully encourage mask-wearing.


The study wasn’t without its limitations. Namely, that identifying symptomatic infections relied on the consent of the participant, which was not always given, potentially making the number of infections much higher. Likewise, only symptomatic individuals were tested for the virus, meaning asymptomatic infections were unaccounted for. 

The study also couldn’t explain their results. Were masks making symptoms less severe by reducing the viral load participants were exposed to? Or were they reducing new infections entirely? According to the Post, the authors are planning further research to answer these questions.



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