A huge randomized trial of communities in Bangladesh appears to provide the clearest evidence to date that wearing a mask regularly can hinder the spread of the covid-19 pandemic. The study found that villages where masks were heavily promoted and became more popular had significantly lower rates of covid-like symptoms and past confirmed infections than villages where mask wear remained low. These improvements were even more pronounced for villages that received free surgical masks rather than cloth masks.
Many data have emerged over the past year and a half to support the use of masks during the covid-19 pandemic, both in the real world and in the laboratory. But it’s less clear exactly what benefit these masks can offer wearers. (and their communities), and there are at least a few studies that have been inconclusive showing a notable advantage.
A problem with interpreting all of this information is that we have relied heavily on observational studies, which can only ever show a correlation between two things, and not establish a cause and effect relationship. There could be other factors that explain both why one city has a higher mask-wearing rate and a lower rate of diagnosed cases than another city, for example, rather than the first one helping to cause the disease. second.
Late last year, however, dozens of scientists partnered with public health organizations and the government of Bangladesh to conduct a massive randomized trial of masks, often considered the gold standard of the proof. And Wednesday they published their research results in a working paper through the nonprofit Research Innovations for Poverty Action.
The study covered 600 villages in a single region of the country with over 350,000 adult residents combined. Similarly matched villages were randomly assigned to two conditions (a pair of villages with similar population density, for example, would go to one condition or the other). In one condition, the researchers and their partners promoted the use of masks through various incentives between November 2020 and January 2021. These incentives included free masks, endorsements by local leaders, and sometimes financial prizes for villages that have achieved widespread use of masks. In two-thirds of intervention villages, the free masks provided were surgical, while one-third received free cloth masks. In the second condition, the researchers simply observed the villages and did nothing to encourage masks during this time.
Residents in the villages where masks were encouraged did start wearing them more, though no individual nudge or incentive seemed to do better than the others. By the end, about 42% of residents in these villages wore masks regularly, compared to 13% of those in the control group. And in these communities, the odds of people reporting symptoms that may have been covid or testing positive for antibodies to the virus declined.
Overall, the average proportion of people who reported symptoms in the weeks following the mask promotions went down by 11% in these villages compared to the control group, and the average number of people having antibodies went down by over 9%. These differences were larger for surgical mask-wearing villages (12% vs 5% for reducing symptoms) and for residents over 60 (35% for reducing infections for older residents in surgical mask-wearing villages).
Some of this effect might not have come directly from the ability of masks to block transmission of the virus. Those who used masks, the study found, were also more likely to practice social distancing. That’s a relevant finding, the authors note, since some people who have argued against mask mandates do so by claiming that masks will only make people act more carelessly. This study suggests that the opposite is true—that masks make us more, not less, conscientious of others.
The findings are not in a peer-reviewed journal as of yet, an important step for validating any research. And they do carry some limitations, as any study does. The study began and ended before the emergence of the Delta variant, for instance, a much more transmissible version of the coronavirus that’s become widespread throughout the world (at the time, the Alpha variant was most prevalent).
Study author Jason Abaluck, a health and behavioral economist at Yale University, told Gizmodo in an email that his team has submitted the paper for publication in the journal Science. On Twitter, Abaluck has addressed other potential caveats of the study. Some have pointed out, for instance, that the authors only found a protective effect from masks for people under the age of 50 in experiencing covid-like symptoms, not in having antibodies (for older people, a reduction in symptoms and antibodies was seen across the board in mask-wearing villages). But Abaluck argument that this may simply be due to the fact that only 40% of people with symptoms chose to be tested, so any estimates in this group may be less precise. And even if the masks somehow had no direct effect on people under the age of 50, they could still reduce the spread of the virus from the youngest to the oldest, so the masking would be. still a net positive at the level of the population.
The authors also say that the masks could likely have a greater effect in slowing the spread of the current pandemic at the population level than they did at the end of the study, given Delta’s higher transmissibility per case. And because they noticed a significant effect after only a modest increase in mask use, the benefits could be even greater with widespread masking.
“Our results should not be interpreted to imply that masks can only prevent 10% of covid-19 cases, let alone 10% of covid-19 mortality,” they wrote. “Our intervention prompted 29 more people out of 100 to wear masks, with 42% of people wearing masks in total. The total impact with near universal masking – perhaps achievable with alternative strategies or stricter enforcement – may be several times greater than our estimate of 10%.
If this hypothesis turns out to be true, it provides further support for models showing that universal mask coverage in places like the United States can still significantly mitigate the impact of the pandemic. A recent forecast from researchers at the University of Washington, for example, valued that universal mask coverage could prevent up to 50,000 deaths by December 1 of this year.
Perhaps more importantly on an individual level, the study also suggests that cloth masks should be phased out as the recommended mask choice and that surgical masks should be the default going forward, the authors say. .
“While sheet masks clearly reduce symptoms, we cannot dismiss the fact that they have no or only little impact on symptomatic SARS-CoV-2 infections,” they wrote.