Experts from Stanford and the University of California joined in the study of more than 350,000 people from villages across rural Bangladesh.
Among the key findings, surgical masks helped reduce COVID-19 cases in adults 60 years and older by more than a third.
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"So I think the main takeaway is that masks are definitely effective in preventing COVID, said lead author, Dr. Ashley Stczynski of Stanford Medicine, "Surgical masks we have more confidence that they are very effective. And, that the benefits are particularly high among these high-risk groups in the older age populations."
Researchers also found that making masks free of charge was key in getting more people to wear them, along with soliciting help from influential leaders and popular athletes.