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MYTH: Face masks are a COVID-19 silver bullet that are the single or best tool for preventing SARS-CoV-2 transmission.

FACT: Face masks are an essential component of a broad array of non-pharmaceutical interventions (NPIs) that prevent the spread of SARS-CoV-2 and include physical distancing, work from home, limiting large group gatherings (particularly indoors), and increased hygiene and sanitation, in addition to widespread and correct face mask usage.

By: Joseph Osmundson
Last updated: Updated July 16, 2020

Face masks are routinely worn in some countries during cold and flu season to prevent transmission of these seasonal viruses. Early in the global COVID-19 pandemic, many countries mandated or encouraged the use of surgical or cloth face masks to slow or prevent transmission in the community. In the U.S., the Centers for Disease Control and Prevention (CDC) and federal officials initially dismissed face mask usage outside of the typical hospital setting before reversing course.

As face mask usage in the U.S. apparently increased, some initial hotspots (such as New York City) were able to control the initial COVID-19 spike. This, along with correlative data between countries with a low rate of COVID-19 infection and a high rate of mask wearing, led to the theory that masks are the most effective tool, or perhaps even are uniquely able, to stop SARS-CoV-2 transmission.

In a recent (and hotly contested) PNAS paper, Renyi Zhang et al., from Mario Molina’s research laboratory at University of California San Diego argue that most COVID-19 transmission is primarily through aerosols and that masks prevent this transmission. In a widely popular Twitter thread, Jeremy Howard analyzed published and unpublished data to come to the same conclusion. The thread was based mostly on optical images of droplets from breath, but did not consider whether any of these droplets contain infectious SARS-CoV-2 particles, leading to widespread criticism that these data were being overinterpreted.

While mask wearing is clearly an important component of a broad COVID-19 response, it’s not yet clear how effective masks (either surgical or cloth) are at preventing SARS-CoV-2 transmission or what the effect of widespread mask wearing is on COVID-19 at a population and epidemiological level. Various mask types have shown a wide (5-95%) range of protection against the spread of various sizes of liquid droplets. Also still unclear are the size of liquid droplet most likely to carry SARS-CoV-2 virions (viral particles) and how many virions are required to initiate a COVID-19 infection.

In the absence of clear causative data, there remains strong indications that widespread mask wearing is a crucial component of a responsible response to the COVID-19 pandemic. However, in this time of diminished trust between scientists, public health officials, and the general public, we must not overinterpret data or make conclusive claims unless they are built upon a foundation of clear and quality data.

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