The Human Studies Cited by the CDC’s Recent Scientific Brief do not Support Community Masking

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On November 10, the CDC published a Scientific Brief arguing the efficacy of cloth masks to control the spread of COVID. The brief was divided into three parts: source control (exhaling), personal protection (inhaling), and mask studies. Megan Mansell at RationalGround addressed the first two parts.  Here, I’ll address the mask studies.

The mask study section of the brief opens by citing an anecdotal study where two mask-wearing COVID-infected hair stylists saw 139 clients (an estimated 136 wore masks), and none of the 67 clients who agreed to be tested were found to be COVID-positive. To even entertain the thought of using such an anecdote to argue for masking hundreds of millions of people is bizarre–more so after seeing that the study offered no details about the physical environment where the hair styling took place.  

Moreover, no statistics regarding general hair stylist-to-client transmission rates or even estimations of such rates are discussed, leaving the unanswered question: what do researchers believe would have been the expected transmission rate without masks? Finally, more than half of the exposed clients were not tested for COVID, and 35 of those were not even contacted. This leaves open the possibility that there were infections, but the infected people chose not to respond to health officials. All in all, it is an interesting occurrence but not worthy of influencing the lives of hundreds of millions of people.

Another study referenced by the CDC’s brief in support of mask mandates describes COVID in Arizona after implementing a mask mandate. The study includes this graph showing a June 17 mask mandate.

The study observed, “In Arizona, decreases in daily COVID-19 cases were observed after … mandated and enforced mask wearing,” among a host of other mitigation efforts. This statement was followed by, “the relationship between mitigation measures and changes in case counts are temporal correlations and should not be interpreted to infer causality.” Which makes one wonder: what’s the point of the study?

Anyhow, if the CDC is going to use such temporal evidence to support the idea of mask effectiveness, they have an ethical duty to also consider the opposite, where mask mandates do not have a temporal relationship to a decline in cases. Ian Miller has posted dozens of such charts at here and here. Cherry-picking one study that happens to have mask-wearing coincide with a case decrease is disingenuous. Also, it is now clear the masks weren’t the driver in Arizona’s case reduction in July since they are still wearing masks and cases have begun to rise again.

Many of the studies cited by the CDC in the brief suffer from the same problem: they were conducted at a particular point in time, and subsequent events don’t support the idea that mask-wearing significantly limits the spread of COVID. One such cited study listed a host of countries that were wearing masks within 31 days of local onset of a COVID outbreak and had a relatively low mortality rate. The assertion was that quick implementation of masks prevented cases from spreading.

The problem with the study is that it was based on data as of May 9, 2020. Since then, the cited European countries have experienced rapid growth in fatalities and are no longer among the countries with low COVID mortality. For example, Czechia is listed as having a fatality rate of 25.8 per million, but by the writing of this article, that had increased to 520 per million in a surge that started in late September. Bosnia and Herzegovina had a similar experience, moving from 31.1 to 520 deaths per million. Moreover, the other countries listed are largely in Africa and East Asia and have generally experienced much lower rates than the rest of the world, for yet to be determined reasons. Meanwhile, other regions, such as South America, have experienced high fatality rates despite having some countries, such as Peru and Argentina, with strict mask-wearing mandates.

In the end, the CDC’s assertion that the cited human studies of masking support the idea that community mask-wearing is an effective strategy against the spread of COVID is unsupported.