Listener Blumie highlighted that I (Nazim) was rather irresponsible in describing the impact of masks on coronavirus transmission rates. I apologize: wearing respiratory masks is absolutely the best way to decrease coronavirus infection rates in everyday situations, along with distancing and regularly washing our hands.
What I was trying to describe, and may have failed to, was the results of the large scale meta-study that compares the findings of 172 published studies on the factors that influence infection rates in real-world settings. Most studies track droplet propagation rates, or have small sample sizes, or perform tests in rather unrealistic laboratory conditions. Some of them even simply press a pipe against a mask and measure the transmission rate. The reviewed 39 studies track (among other things) the impact of masks on real-world infection rates. They provide this finding: “Face mask use could result in a large reduction in risk of infection (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD −14·3%, −15·9 to −10·7; low certainty).” Those numbers are in line with what the CDC said, and also with my approximations. RD stands for reduction: it’s the difference between 17.4% (the risk of getting infected when not wearing a mask) and 3.1% (the risk of getting infected with a mask). That’s a reduction of over 80%, as the CDC says.
I was also mistaken when I said “low confidence interval.” I was erroneously referring to the certainty level, and here’s what the researchers say on that subject:
“The effect was very large, and the certainty of evidence could be rated up, but we made a conservative decision not to because of some inconsistency and risk of bias; hence, although the effect is qualitatively highly certain, the precise quantitative effect is low certainty.”
So, while it’s still the best defense we can use to decrease infection rates, in a legislative context, there is plenty of room to discuss what reasonable restrictions one should impose. However, as Blumie reminded me, masks (along with distancing and regularly washing your hands) are still the best everyday defense to decrease infection rates.
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