The Science Behind Wearing Masks

Miller Report for the Week of September 22, 2020

By William Miller, MD – Chief of Staff at Adventist Health – Mendocino Coast Hospital

Masks are the most effective strategy we have for controlling the overall spread of COVID through our communities, perhaps even more important than handwashing and social distancing.  We are beyond the point where surveillance testing and contact tracing would be useful and we haven’t been able to keep up with the resources needed to do either.  Also, it remains unclear whether any vaccine will be all that effective or that the immunity such a vaccine grants will be sustained.  That is not to say that these other measures are not important, just that masks are perhaps the single most important element in our strategy.  So, let’s take a closer look at masks.

There is tremendous scientific support for wearing masks to prevent transmission of all sorts of respiratory infections, not just COVID.  There are two studies in particular that are very convincing on how effective they can be.  In July, the CDC reported on two hairstylists in Missouri who were infected with SARS-2, the virus that causes COVID (MMWR / July 17, 2020 / 69(28);930-932).  These two had been tested because they both had respiratory symptoms consistent with COVID and remember that people are most contagious when symptomatic.  It took 8 days for the test results to come back and during those 8 days the two continued to work, seeing 139 clients.  Both hairstylists and all clients wore masks during their visits with about half wearing cloth masks and half wearing medical grade masks.  By nature of the visits, these all involved close personal contact (within 6 feet) for between 15 and 45 minutes (with an average of about 19 minutes).  When the two hairstylist’s results came back positive, public health officials traced all 139 clients and tested 109 of them.  All 109 tested negative.  All 139 were followed regardless of testing and none developed symptoms.  Meanwhile, several family members of the hairstylists did come down with COVID supporting that the two were infectious.

Another study involved a married couple who flew from Wuhan, China, to Toronto, Canada, on January 22^nd, published in the Canadian Medical Association Journal (CMAJ / April 14, 2020 / 192(15);E410).  The husband was symptomatic during the 15-hour flight with a cough and the wife became symptomatic the day after landing.  Both tested positive for SARS-2.  Both wore masks during the entire flight, as did all other passengers and crew.  Health officials tested and monitored the 25 passengers who sat nearest the couple and the flight crew.  All of these close contacts repeatedly tested negative and none developed symptoms.

The reason that masks work so effectively is that the virus is not appreciably spread as individual particles, but instead in the form of respiratory droplets made of saliva, mucus or other respiratory secretions.  These droplets, while mostly invisible to the eye are none-the-less large enough to be trapped in the fabric of a mask.  They are released into the air by talking, laughing and singing, but especially large amounts of droplets are created when we cough or sneeze.  A study published in the New England Journal of Medicine in May used high spread photography to demonstrate that simply speaking a single phrase generates hundreds of respiratory droplets (NEJM / May 21, 2020 / 382;2061-2063).

The highest viral load is around the time when symptoms first develop including just before, hence the reason we get the symptoms.  When we combine high viral load with symptoms such as coughing, then it is easy to see why people are most contagious at that time.  However, even people who never develop symptoms can spread the virus since viral loads are potentially high enough to be contagious without symptoms. Thus, it is important that steps be taken by all people regardless of symptoms.

Of all people who get infected by SARS-2, about 80% will have no symptoms or minimal symptoms.  Only about 20% get really sick and of those, only about a third require hospitalization.  When we look at the cases, it appears that at least half of all people with COVID got it from someone who never developed symptoms.  These points underscore the importance of all persons wearing masks, even if they don’t think that they are infected.

The strategy for asking everyone to wear masks is to prevent infected people from spreading it to others.  This strategy is important because we are trying to minimize the risk of spreading the disease and with time, the disease will fade away.  There is also strong evidence that if between 85-100% of all people wore masks consistently, we could probably do away with restricting businesses and closing down our economy.  If you think about it, mask wearing as a strategy is far less costly than people losing jobs and businesses going under.  

I do think that most people are now wearing masks when they are out and about in town and pretty much no one is going into stores unmasked.  However, now we need to look more closely at social gatherings and that includes large family gatherings.  Once again, if everyone wears a mask then essentially no one will get this disease.

For next week’s Miller Report, I will go into detail on the research comparing the different types of masks such as medical grade versus cloth and what types of cloth are more effective than others.

If you would like to read more on the subject of masks as an important strategy for prevention, I recommend these two on-line review articles which might be helpful as they are filled with many useful links.  They are: https://theconversation.com/can-people-spread-the-coronavirus-if-they-dont-have-symptoms-5-questions-answered-about-asymptomatic-covid-19-140531  and https://www.ucsf.edu/news/2020/06/417906/still-confused-about-masks-heres-science-behind-how-face-masks-prevent .