Distinct regimes of particle and virus abundance explain face mask efficacy for COVID-19

Abstract:

Airborne transmission is an important transmission pathway for viruses, including SARS-CoV-2. Regions with a higher proportion of people wearing masks show better control of COVID-19, but the effectiveness of masks is still under debate due to their limited and variable efficiencies in removing respiratory particles. Here, we analyze experimental data and perform model calculations to show that this contrast can be explained by the different regimes of abundance of particles and viruses. Upon short-term exposure, respiratory particles are usually in a particle-rich regime, but respiratory viruses are often in a virus-limited regime where the numbers of viruses inhaled by susceptible people are below or close to the infectious dose. Considering the currently available knowledge of virus loads, emission rates, airborne virus concentrations, and their variability, we find that the average abundance of airborne SARS-CoV-2 is in a virus-limited regime for both aerosol and droplet transmission in most environments. The characteristics of this virus-limited regime explain the efficacy of face masks and enable synergetic effects of combining masks with other preventive measures such as ventilation and social distancing to reduce the overall risk of infection. We also find that the concentrations of virus emitted into the air have a large variability between individuals (3 to 4 orders of magnitude increase from the 5th percentile to the 95th percentile), which is essential in the assessment of infection risk and mask efficacy.