When COVID-19 started to spread from China to other countries in late January, it seemed like Thailand--a popular destination for Chinese tourists, including thousands who had traveled from Wuhan that month--might be hit hard. But as of early June, the country has had only a little more than 3,000 confirmed cases and 58 deaths. In the U.S., the death rate per capita is more than 450 times greater.

Several factors likely slowed the spread of the virus in Thailand, including partial lockdowns, contact tracing, and community health volunteers who helped track the disease throughout the country. But one seemingly key factor was the use of masks, which became prevalent there even as the World Health Organization was saying that they weren't needed (the WHO has since reversed course). Even now, with only a handful of new cases being reported, 95% of Thailand's residents report wearing masks when they're in public. A new study that looked at global data found that countries that required masks early in the outbreak, or where people quickly adopted them, as in Thailand, have had death rates 100 times lower than projected.

"We looked at the data from 198 countries around the world, and we looked at mortality from coronavirus," says lead author Christopher Leffler, a physician and professor at Virginia Commonwealth University. "What we found is that the countries that introduced masks quickly--that is, before the outbreak had much of a chance to spread within their country--had a much lower mortality."