What’s the evidence that by wearing face masks we are protecting one another from SARS-CoV-2 infection?

June 22, 2020

Contributed by Dr. Ashley Haase, MD Regents’ Professor and Head, Department of Microbiology and Immunology; Professor of Medicine Infectious Diseases and Internal Medicine

I know this has been an on-again, off-again issue, so to speak, but wearing masks may have reduced the number of infections by over 78,000 in Italy from April 6 to May 9, and over 66,000 in New York City from April 17 to May 9.

In a paper in the PNAS on “Identifying airborne transmission as the dominant route of spread of COVID-19,” Renyi Zhang Mario J. Molina et al.  arrive at this conclusion by  comparing the rate at which new infections are acquired before and after wearing face masks is mandated. Quarantine, isolation and city lockdown measures were implemented on March 9 in Italy,  and similar guidelines in the United State for quarantine, distancing and isolation were issued by the federal government on March 16. These measures alone didn’t prevent a continuous rise in US infected numbers, but the rate of increases in new cases between April 17 and May 9 in NYC and the US, when the only difference in regulatory measures was mandated face covering in NYC, decreased ~3% per day in NYC and increased by ~0.3% per day in the US as a whole. This difference attributed to face covering translates to 66,000 fewer infections in NYC. In Italy, the rate of new infections also started to decline after mandated face covering on April 6 with an estimated reduction of 78,000 new infections to May 9 (Fig 1).

successive implementation of face covering graph

Fig 1. Successive implementation of face covering in Italy and NYC rather than simultaneous implementation of all control measures in China enable Zhang et al to tease out the separate impact of the addition of face covering (black circle and symbol) on reducing the rate of new infections-where the red and pink lines bend down from the dotted line represent the continuing predicted rates.

The successive implementation of face covering in addition to distancing, isolation, quarantine in place everywhere in reducing new cases points to the importance of airborne transmission, particularly in enclosed environments, and several important questions for future research (Fig 2):


Fig 2 Spread of COVID 19

  • Number and size distributions of nascent aerosols?
  • Viral load per particle from coughing/sneezing?
  • Deposition of aerosols along the respiratory tract?
  • Minimum dose of airborne viruses required for infection?

Most importantly, we need to evaluate the performance of face masks to filter out airborne viruses relevant to human atomization and inhalation. Encouraging work along these lines has recently been reported by Leung et al., who show that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.

I end this piece by affirming the importance of masks in preventing transmission and by reiterating the plea in Zhang et al. that integration between science and policy is crucial to formulating effective emergency responses by policy makers and preparedness by the public for the current and future public health pandemics. Please, everyone, do your part to contribute to disease prevention.