Face Masks Against COVID-19: An Evidence Review

Abstract

The science around the use of masks by the general public to impede COVID-19 transmission is advancing rapidly. Policymakers need guidance on how masks should be used by the general population to combat the COVID-19 pandemic. In this narrative review, we develop an analytical framework to examine mask usage, considering and synthesizing the relevant literature to inform multiple areas: population impact; transmission characteristics; source control; PPE; sociological considerations; and implementation considerations. A primary route of transmission of COVID-19 is via respiratory droplets, and is known to be transmissible from presymptomatic and asymptomatic individuals. Reducing disease spread requires two things: first, limit contacts of infected individuals via physical distancing and other measures, and second, reduce the transmission probability per contact. The preponderance of evidence indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts. Public mask wearing is most effective at reducing spread of the virus when compliance is high. The decreased transmissibility could substantially reduce the death toll and economic impact while the cost of the intervention is low. Given the current shortages of medical masks we recommend the adoption of public cloth mask wearing, as an effective form of source control, in conjunction with existing hygiene, distancing, and contact tracing strategies. Because many respiratory droplets become smaller due to evaporation, we recommend increasing focus on a previously overlooked aspect of mask usage: mask-wearing by infectious people ("source control") with benefits at the population-level, rather than mask-wearing by susceptible people, such as health-care workers, with focus on individual outcomes. We recommend that public officials and governments strongly encourage the use of widespread face masks in public, including the use of appropriate regulation.

The Risk of Indoor Sports and Culture Events for the Transmission of COVID-19 (Restart-19)

“…In super-spreading events described in the literature, masks were not used ( 20 ). In the meantime, effects of masks on the reduction of transmission are meanwhile generally accepted ( 21, 22 ). Aerosols are of special concern during halves when visitors stay in their seats and exposition time can accumulate.…”

Hydrating the Respiratory Tract: An Alternative Explanation Why Masks Lower Severity of COVID-19 Disease

“…Here, we propose that the increased humidity of air inspired through face masks is responsible for the lower disease severity of mask wearers [27, 28]. In contrast to the primary function of masks to prevent virus from entering the respiratory tract of another person [45, 46], this additional benefit applies after a virus-containing particle lands on the surface of the respiratory tract: reduced dehydration limits impairment of the innate immune system [14] while improving mucociliary clearance [16, 17], with both these factors reducing infection probability. If an infection does occur, humidification may limit its further spread through the lungs by lowering the generation of virus-containing breath droplets that could lead to self-inoculation elsewhere in the lungs [18, 19].…”

A Multi-Criteria Framework for Pandemic Response Measures

“…Contact tracing combined with public disclosure of active cases' location seems to lower the number of deaths, having 50% lower economic costs than full lockdown (24). A comparatively cheap measure is to wear masks and some evidence suggests that wearing such can reduce transmissibility and be highly effective when compliance is high, at the same time substantially reducing both the death toll and the economic impact (25). Wearing them at 96% alone could flatten an epidemic growing at a rate of 0.3/day by bringing down the reproduction number from an original value of 3.68 to 1.…”