The study involved launching pro-mask campaigns in some Bangladeshi villages, but not others, and the authors made two key findings. First, they determined that the public health interventions nearly tripled mask usage from 13 percent to 42 percent. Secondly, they discovered -- by conducting sero-surveys backed up by interviews about COVID-19 symptoms and medical history -- that masks did their job and reduced symptomatic infections in the communities that were subject to the campaigns by 9.3 percent. Jason Abaluck, an economist at Yale University who helped lead the study, told The Washington Post that figure would probably be higher if masking was universal. 

There were a few other key notes in the study. Surgical masks were found to be particularly effective, while the jury is still out on cloth masks. And they were more effective in people older than 50, which could be explained by a few factors, including that young people were less likely to be symptomatic either way. They also may have been less compliant when it comes to masks.