The CDC's Director Implies That Face Masks Are More Effective Than Vaccines at Preventing COVID-19 Infection

Rochelle Walensky seems to be relying on a laboratory study that did not measure infection risk.


Are face masks more effective than vaccination at preventing COVID-19 infection? That is the implication of a new public service announcement from the Centers for Disease Control and Prevention (CDC) featuring Rochelle Walensky, the agency's director.

"The evidence is clear," Walensky says in the 37-second video, which she posted on Twitter last Friday. "Masks can help prevent the spread of COVID-19 by reducing your chance of infection by more than 80 percent, whether it's an infection from the flu, from the coronavirus, or even just the common cold. In combination with other steps like getting your vaccination, hand washing, and keeping physical distance, wearing your mask is an important step you can take to keep us all healthy."

If wearing a mask reduced your risk of infection by "more than 80 percent," as Walensky seems to be saying, that safeguard would be amazingly effective. Such a risk reduction would be higher than the effectiveness rates found in several real-world studies of mRNA vaccines. In six studies conducted when the delta variant was dominant, vaccination was associated with infection reductions ranging from 54 percent to 85 percent. The effectiveness rate was 80 percent or less in five of those studies, although the reductions in symptomatic cases, severe disease, and hospitalization were bigger (an important point to keep in mind when assessing the benefits of vaccination).

What is the source of Walensky's startling claim that masks prevent infection better than vaccines do? I asked the CDC but have not heard back yet.* Judging from the CDC's summary of the evidence in favor of face masks, however, Walensky is relying on a laboratory study that was reported in the journal Science Advances in September 2020.

The researchers used a camera to record laser-illuminated respiratory droplets emitted by speakers who wore 14 kinds of face coverings, ranging from N95 respirators and surgical masks to bandanas and neck gaiters. The valveless N95 mask was 99.9 percent effective at retaining "droplet sizes larger than 0.5 μm" (the estimated detection limit), while the neck gaiter seemed worse than useless, apparently because it broke larger droplets into smaller ones. In between, three-layer surgical masks and several kinds of cloth masks reduced the number of droplets detected by more than 80 percent. Based on those results, the CDC's summary says "upwards of 80% blockage has been achieved in human experiments that have measured blocking of all respiratory droplets."

While that gloss is a bit misleading given the detection limit noted by the researchers, it is still a far cry from Walensky's assertion that masks "reduc[e] your chance of infection by more than 80 percent." It is fair to present this study as evidence that face masks can help reduce the risk of COVID-19 transmission. But Walensky is doing more than that: She is trying to quantify the real-world impact of masks based on a laboratory study that did not measure it.

There are several reasons to be wary of such conclusions, including the gap between stylized laboratory conditions and messy reality. In this study, one person wearing a mask spoke through a hole in a box. He recited the phrase "Stay healthy, people," but he did not cough, sneeze, shout, or sing. The masks were clean and worn properly, which is not always true in real life. And although the study found that the type of cloth mask had an important impact on its performance, Walensky's message implicitly assumes that everyone will choose the most effective models.

Worse, Walensky's ambiguous statement could be interpreted to mean that people who wear masks thereby reduce their own risk of catching COVID-19 by "more than 80 percent." Walensky reinforces that impression in her tweet, where she says "wearing a mask" will help you "stay healthy." The study, which looked at droplets emitted by mask wearers, provides no basis for such a claim.

At the same time, Walenksy's tweet seems to qualify her estimate, saying "masks can help reduce your chance of #COVID19 infection by more than 80%" (emphasis added). That could be true even if masks are much less effective than Walensky says in the video, assuming you are also vaccinated.

Given the gap between the study and Walensky's interpretation of it, she should have avoided the false precision of that "more than 80%" number altogether. You might think a public health official who has repeatedly caught flak for distorting COVID-19 research would have learned to be more careful when presenting scientific findings to the public. Walensky's mask hyperbole not only further undermines her credibility; it implicitly makes masking look superior to vaccination as a tool for avoiding COVID-19 and preventing its spread, which is hardly a message conducive to public health.

*Update, November 10: Today I received a reply from the CDC that did not cite any specific research to back up Walensky's claim. The email, which may or may not have involved an actual human, consisted of boilerplate advice about masks and links to several CDC publications, including the research summary that cites the Science Advances article.