Health News Digest


October 12, 2020

The Latest On Masks

By Michael D. Shaw

The matter of masks during the COVID-19 pandemic has been interesting, to say the least. Details of exactly how, and under what conditions they are supposed to be donned vary widely. Never mind individual adherence to the policies, or how the masks fit. At my health club, for example, masks are supposed to be worn at all times, except when one is in the act of exercising. Yet, some guys wear them constantly, only removing them when they shower.

But beyond all these reactions and interpretations, the question remains: How well do masks work in fending off SARS-CoV-2? We look at the latest studies…

In an July, 2020 editorial appearing in JAMA, three physicians from the CDC presented their arguments in favor of universal masking to prevent SARS-CoV-2 transmission. Two case studies were cited.

The first one examined 9850 healthcare workers associated with the Mass General Brigham healthcare system. More details are given in the linked article, but we can summarize the results:

Prior to the institution of the mask policy, new cases among healthcare workers who had either direct or indirect patient contact were increasing exponentially. After the policy was put into place, however, the proportion of symptomatic healthcare workers who tested positive for COVID-19 steadily declined. See graphic. Confounding factors could include restrictions on elective procedures, social distancing measures, and increased masking in public spaces.

The second one concerned two Springfield, MO hair stylists. On May 12, 2020, Stylist A developed respiratory symptoms and continued working with clients until May 20 due to a positive test for SARS-CoV-2. Stylist B had been exposed to A, developed symptoms on May 12, and worked with clients until May 22 when a positive test was also received. All told, 139 clients were exposed, but both stylists wore masks, as did 98% of the interviewed clients. While not all the clients agreed to be tested, among those who did, none tested positive for the virus.

We now turn to a few studies highlighted in a Nature news feature. A preprint study in which data was examined from 200 countries concluded that societal norms and government policies supporting the wearing of masks by the public, as well as international travel controls, are independently associated with lower per-capita mortality from COVID-19.

There was an animal study in which infected hamsters were or were not separated by a screen made from mask material. Absent the barrier, about two-thirds of the uninfected animals caught SARS-CoV-2. But only about 25% of the animals protected by mask material got infected, and those that did were less sick than their mask-free cohorts.

The news feature wraps up by acknowledging the lack of truly definitive data, and then states: “Nevertheless, most scientists are confident that they can say something prescriptive about wearing masks. It’s not the only solution, but…it is a profoundly important pillar of pandemic control.” As virologist Paul Digard puts it: “Masks work, but they are not infallible. And, therefore, keep your distance.”

We conclude with a brief look at a series of charts that have become popular. The charts show the progress of daily COVID-19 cases over time, with indicators for when lockdowns started, and when masks were mandated. Superficially, these have been interpreted as showing that masks don’t do anything, but a serious fallacy is in play here.

Consider Cum hoc ergo propter hoc (literally translated as “With this, therefore because of this”) usually rendered as “Correlation does not imply causation.” In this case, the mere association of mask mandates with a measurement of daily COVID-19 infections does not prove anything. The point certainly cannot be made that the masks had no effect on the rising rates unless one had control data (which of course does not exist) showing what infections would be like without mask mandates in those very same locations.

In fact, the closest thing to a true controlled experiment, but for the asynchrony, is the previously mentioned Mass General Brigham study. Or to paraphrase what was already said, wear your masks—imperfect though they may be—and also maintain social distancing when you can.