What evidence do we have that wearing a mask is effective in preventing COVID-19?

One category of evidence comes from laboratory studies of respiratory droplets and the ability of various masks to block them. An experiment using high-speed video found that hundreds of droplets ranging from 20 to 500 micrometers were generated when saying a simple phrase, but that nearly all these droplets were blocked when the mouth was covered by a damp washcloth. Another study of people who had influenza or the common cold found that wearing a surgical mask significantly reduced the amount of these respiratory viruses emitted in droplets and aerosols.

But the strongest evidence in favor of masks come from studies of real-world scenarios. “The most important thing are the epidemiologic data,” said Rutherford. Because it would be unethical to assign people to not wear a mask during a pandemic, the epidemiological evidence has come from so-called “experiments of nature.”

A recent study published in Health Affairs, for example, compared the COVID-19 growth rate before and after mask mandates in 15 states and the District of Columbia. It found that mask mandates led to a slowdown in daily COVID-19 growth rate, which became more apparent over time. The first five days after a mandate, the daily growth rate slowed by 0.9 percentage-points compared to the five days prior to the mandate; at three weeks, the daily growth rate had slowed by 2 percentage-points.

Another study looked at coronavirus deaths across 198 countries and found that those with cultural norms or government policies favoring mask-wearing had lower death rates.

Two compelling case reports also suggest that masks can prevent transmission in high-risk scenarios, said Chin-Hong and Rutherford. In one case, a man flew from China to Toronto and subsequently tested positive for COVID-19. He had a dry cough and wore a mask on the flight, and all 25 people closest to him on the flight tested negative for COVID-19. In another case, in late May, two hair stylists in Missouri had close contact with 140 clients while sick with COVID-19. Everyone wore a mask and none of the clients tested positive.