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 last September.
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 other 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.
Science!