Here’s the evidence reported by Stanford Medicine »

A large, randomized trial led by researchers at Stanford Medicine and Yale University has found that wearing a surgical face mask over the mouth and nose is an effective way to reduce the occurrence of COVID-19 in community settings.

It also showed that relatively low-cost, targeted interventions to promote mask-wearing can significantly increase the use of face coverings in rural, low-income countries. Based on the results, the interventional model is being scaled up to reach tens of millions of people in Southeast Asia and Latin America over the next few months.