Results from a massive study in Bangladesh unequivocally show that surgical masks reduce the spread of SARS-CoV-2, scientists say. The results — from the highest-quality, gold-standard type of clinical trial, known as a randomized controlled trial — should “end any scientific debate” on whether masks are effective in battling the spread of COVID-19, Jason Abaluck, an economist at Yale and one of the authors who helped lead the study, told The Washington Post. “This is an incredibly challenging but important study to pull off,” Megan Ranney, an emergency medicine physician and a professor at Brown University who was not part of the study, told the Post. “Anti-mask people keep saying, ‘Where’s the randomized controlled trial?’ Well, here you go.” For the past year and a half, scientists have said that masks reduce the spread of the virus. But it’s very difficult to study how much masks help to curb transmission in the real world, where not everyone is masking, using the same quality of masks or even wearing masks properly. Observational studies, which simply compare mask wearing behaviors to infection rates in different areas, can be muddied by so many other factors. Randomized trials — in which people are randomly assigned to receive a medical intervention or not — are the most robust form of evidence. But those are expensive and difficult to conduct, especially for a behavior like masking. In the new study, researchers from Bangladesh and the U.S. tested the effectiveness of mask promotion and usage across 600 villages in Bangladesh. The study, which involved more than 342,000 adults, is the largest randomized trial ever conducted on mask usage, according to the Post. The study was posted as a preprint to the Innovations for Poverty Action nonprofit website on Sept. 1 while it is being peer-reviewed for publication in the journal Science, according to the Post.