IN THE ARSENAL of measures to prevent covid-19, face masks hold a special place. They are cheap, easy to use and can be deployed almost immediately. Laboratory studies have shown that face coverings made of various materials block large amounts of the SARS-CoV-2 virus. But there is a dearth of data on how effective masks are in preventing actual covid-19 cases in real-life conditions, where masks are worn sporadically and, all too often, incorrectly.

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A large study published last month by a consortium of researchers from several think-tanks and American universities provides some useful evidence on the matter. It was conducted in Bangladesh, an unusually densely populated country, at a time when SARS-CoV-2 was circulating widely. It enrolled nearly 350,000 people in 600 villages. Half of the villages were randomly picked to receive free supplies of either cloth or surgical masks regularly. They were distributed in a variety of places including markets, mosques and homes. Instructions were provided on how to wear them properly. Community leaders and other influential figures in those villages encouraged mask-wearing. The random assignment of a large number of villages to each study group ensured that, on balance, differences in mask-wearing and covid-19 cases were not because of pre-existing differences between villages that received masks and those that did not.

Over the course of eight weeks observers stationed in public places around the villages recorded the frequency of mask-wearing. In those villages selected to receive masks and training, proper mask wearing increased to 42% of people counted. In those that didn’t the rate was only 13% of people counted. The prevalence of covid-19 cases in villages that received cloth masks was 8% lower than in villages that received none. And in the villages with surgical masks the rate was 14% lower.

The results were most striking among older people, who are the most likely to die of covid-19. The distribution of surgical masks resulted in a striking 35% reduction in confirmed symptomatic cases among people aged 60 or older. Although these results were from rural Bangladesh, they suggest that masks can be effective elsewhere tooeven when fewer than half of people wear them correctly. Saving lives has never been so easy on the wallet.