How Effective Are Cloth Face Masks?

A summary of the scientific literature on the effectiveness of masking, both against respiratory infection generally and against COVID-19.Since the emergence of the COVID-19 pandemic, many public health officials, political and social leaders, and major news media have urged people to wear face masks to reduce the transmission of the SARS‐​CoV‑2 virus. In the United States, masking has become a flashpoint of controversy, with school boards and local governments facing fervent — and sometimes even threatening — demands either for or against mask mandates.By September 2020, the U.S. government had distributed 600 million facemasks for use by the public as part of its response to the pandemic. At the local level, 32 states and numerous municipalities implemented mask mandates at some point, and some political and social figures called for a nationwide mask mandate. At the height of the pandemic, New York City instituted a $1,000 fine for those who refused to wear face masks in public, and then–presidential candidate Joe Biden proclaimed in a speech, “Wearing masks is not a political statement, it is a scientific imperative.” Over 40% of the global population lives in countries that at one time or another mandated mask‐​wearing in public areas to fight the pandemic.Yet, there is little consensus that masking — at least as commonly practiced in the United States, using cloth masks — is effective at suppressing various types of respiratory infection. In the surgical operating room context, a review by the Cochrane Collaboration — a widely respected nonprofit that provides comprehensive, evidence‐​based summaries on various medical topics — found “no statistically significant difference in infection rates between the masked and unmasked group in any of the trials.” Another Cochrane review, of masking and influenza‐​like illness, found “that wearing a mask may make little or no difference to the outcome of influenza‐​like illness … compared to not wearing a mask.”Given that background, the World Health Organization was initially skeptical of encouraging the general public to wear cloth masks in an effort to slow the pandemic. The WHO’s initial COVID-19 guidelines stated that “cloth (e.g., cotton or gauze) masks are not recommended under any circumstance,” and a subsequent update noted “the widespread use of masks by healthy people in the community setting is not yet supported by high quality or direct scientific evidence.”So, are masking requirements just “public health theater,” providing baseless assurance to a fearful public? Or has new evidence emerged to confirm the belief that masks — or, at least, the cloth masks that are commonly used — reduce respiratory virus transmission? Below is a summary of the scientific literature on the effectiveness of masking, both against respiratory infection generally and against COVID-19.Masks Interrupt Droplet Transmission, but Does That Slow COVID?Among the hallmark memes of the COVID-19 pandemic have been videos using special lighting and high‐​speed photography to visualize the dramatic differences in droplets emitted when a person sneezes with and without a mask. As if subtitling the visuals, an article in the New York Times explained that mask fibers “create a haphazard obstacle course through which air — and any infectious cargo — must navigate.”Such vivid imagery promotes a misconception that the emission of infectious particles primarily occurs during forceful expiration such as sneezing. In fact, respiratory particles are emitted even when breathing. Little evidence suggests that frequent public sneezing during the pandemic has been a key driver of the virus’s spread.It is debated whether larger “droplets” (greater than about 10 micrometers) or smaller “aerosols” containing the virus are more infectious. But aerosol transmission has been demonstrated or is considered likely for other respiratory infections such as H1N1 influenza, Middle East Respiratory Syndrome (MERS), respiratory syncytial virus (a common virus among infants), and the 2003 SARS virus. When smaller particles evaporate, they can stay suspended in the air for long periods of time and be inhaled, potentially causing infection deeper in the respiratory tract and at lower concentrations. SARS‐​CoV‑2 viral particles have been detected in low‐​touch areas such as under beds and in air samples taken from hallways outside patient rooms, consistent with sustained aerosol distribution.The greater the role of aerosols in spreading SARS‐​CoV‑2, the less important is the filtering capability of masks, because exhaled air easily flows around a mask’s edges. The extent to which droplets penetrate a mask has not been established as a reliable surrogate for the prevention of disease transmission.Randomized Controlled Mask Trials and COVID-19The best evidence to establish the effectiveness of cloth face masks would be from cluster‐​randomized controlled trials (RCTs) showing that communities tasked with wearing cloth masks have lower viral spread than those assigned to not wear cloth masks, with high participation and protocol adherence. The endpoint of such studies should be laboratory‐​confirmed SARS‐​CoV‑2 infection and not just symptoms of illness, because mask wearing could affect a user’s perception of symptoms, creating bias.