Every infectious disease has a reproduction rate, called R. When it’s 1.0, that means the average infected person infects one other person. The 1918 pandemic flu had an R of 1.8—so one infected person infected, on average, almost two others. COVID-19’s rate, in the absence of measures such as social distancing and masks, is at least 2.4. A disease dies out if its R falls under 1.0. The lower the number, the faster it dies out.

The effectiveness of mask-wearing depends on three things: the basic reproduction number, R0, of the virus in a community; masks’ efficacy at blocking transmission; and the percentage of people wearing masks. The blue area of the graph below indicates an R0 below 1.0, the magic number needed to make the disease die out.

Models show that if 80 percent of people wear masks that are 60 percent effective, easily achievable with cloth, we can get to an effective R0 of less than one. That’s enough to halt the spread of the disease. Many countries already have more than 80 percent of their population wearing masks in public, including Hong Kong, where most stores deny entry to unmasked customers, and the more than 30 countries that legally require masks in public spaces, such as Israel, Singapore, and the Czech Republic. Mask use in combination with physical distancing is even more powerful.