Why we should wear face masks to tackle COVID-19

Infectious disease modelling is helping us understand how to tackle the COVID-19 pandemic. A key concept involves the basic reproductive ratio R, that compares the rate of new infections per person infected, to the rate of recovery1. To stop an epidemic it is necessary to modify R so that it is less than 1, at which point the number of new infections falls exponentially toward zero with time. Another basic but less appreciated result, is that the proportion of susceptible individuals subsequently infected by the epidemic is less than 1 and determined by R (see figure 1) 1. If R is lower there will be fewer total infections, and as R approaches 1 the proportion of infections falls increasingly fast to zero. These results remain approximately true when the proportion of recovered individuals is small compared to 1/R. As a result, all sustained efforts that reduce the rate of infection will also reduce the total number of people who will get the disease. For example, reducing R from 4 to 3 will lead to 4% fewer infections. In a population with 100 million susceptible individuals, in which roughly 1 in 100 infected people were to die, a 4% reduction would equate to saving around 40000 lives. As R gets closer to 1 the rate of drop is amplified, with a drop of R from 2.5 to 1.5 leading to 31% fewer infections, with a similar reduction in the proportion of severe infections and deaths. This is why cumulative efforts to improve hygene and reduce transmission of disease really matter. The importance of wearing face masks is uncertain due to limited systematic studies2, 3. The most rigorous collection of data regarding transmission of influenza-like diseases (not yet peer-reviewed …