When do you need a mask?

May 28, 2020 By Alan Salzberg

A recent op-ed in the New York Times (https://www.nytimes.com/2020/05/27/opinion/coronavirus-masks.html) discusses how to get more people to wear masks.  However, it glosses over an important question: is it necessary?  In many cases, the answer is no.  

The Times article states that "new scientific evidence is demonstrating the efficacy of masks in the fight against the coronavirus."    

Let's explore the evidence cited in the Times, which is in this Nature article (https://www.nature.com/articles/s41591-020-0843-2?ContensisTextOnly=true).  The research discussed in the Nature article is limited to  people who are ill (i.e., with symptoms) and looks at use of surgical (not cloth) masks.  Only 4 people in the study did not cough during a 30-minute monitoring session, and no virus was emitted from these individuals.  Of those who did cough, only a minority emitted any detectable virus after 30 minutes of breathing and coughing.  While there was no detectable difference in the number emitting virus through droplets from coronavirus (not COVID-19) sufferers, there was a substantial difference (18 of 65, or 28%, without a mask versus 7 of 65, or 11%, with a mask) when flu and common cold sufferers were included. [Because COVID-19 is thought to transmit mainly through droplets and not aerosal, I do not focus on that part of the study, but note that while aerosal emission is higher, it is still below 50%, for both those with and without a mask]  

So what does this say?  Ignoring the margin of error and potential lack of representativeness of the study, about 28% of those coughing and not wearing a mask will emit enough virus in 30 minutes to be detectable whereas 11% of those coughing with a mask will emit enough virus to be detectable.  

This result seems to indicate the WHO's guidance (https://www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public/when-and-how-to-use-masks) to wear a mask if you are coughing or sneezing (or caring for someone who is sick) is appropriate.  This result does not support the CDC's guidance that recommends wearing a cloth mask when social distance cannot be maintained.

In addition to finding that the majority did not emit any detectable virus, the Nature article also found that little virus was shed even by those who did emit the virus: "For those who did shed virus in respiratory droplets and aerosols, viral load in both tended to be low (Fig. 1). Given the high collection efficiency of the G-II (ref. 19) and given that each exhaled breath collection was conducted for 30 min, this might imply that prolonged close contact would be required for transmission to occur, even if transmission was primarily via aerosols..."

These findings regarding transmission are consistent with other studies.  Erin Bromage's blog provides some good discussion and citations here: https://www.erinbromage.com/post/the-risks-know-them-avoid-them?fbclid=IwAR09XdkNY2SZBNyK9oCbKXGMAYBJ1oQiryulWNLjDyrmrOPcq8CrQXLzni0.

What's the takeaway?

You are fairly safe with or without a mask in short interactions with sick people (i.e., grocery store, quick shopping trip, or walking by someone on the sidewalk), whereas you are potentially at risk (i.e, perhaps more without than with a mask but at risk either way) in long interactions (restaurant, work, or  a movie/play/sporting event).