High efficacy of face masks explained by characteristic regimes of airborne SARS-CoV-2 virus abundance

Airborne transmission by droplets and aerosols is important for the spread of viruses and face masks are a well-established preventive measure, but their effectiveness for mitigating COVID-19 is still under debate. We show that variations in mask efficacy can be explained by different regimes of virus abundance. For SARS-CoV-2, the virus load of infectious individuals can vary by orders of magnitude, but we find that most environments and contacts are in a virus-limited regime where simple surgical masks are highly effective on individual and population-average levels, whereas more advanced masks and other protective equipment are required in potentially virus-rich indoor environments such as medical centers and hospitals. Due to synergistic effects, masks are particularly effective in combination with other preventive measures like ventilation and distancing.