Thread by F. Perry Wilson, MD MSCE
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- Nov 14, 2022
- #Pandemic
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Brief thread on that "masks in schools" article in @nejm. It's a very well done article that I think many people have not actually read...
1/N
1/N
The setup is starightforward. Cowger et al have access to data on 300,000+ students and teachers COVID tests in 70+ Eastern Mass public school districts. (2/N) 
They know when these various districts lifted their mask mandate. A bunch did right at the end of Feburary 2021 (1) when the state mandate was lifted, a bunch a week later (2), a bunch a week after that (3), and 2 (Boston / Chelsea - Black) held out for the full time study. 3/N 
The rates of COVID were pretty similar before the Mandate was lifted (prior to the dotted line) but diverge thereafter. Some noise around the Omicron wave when so many people were infected. (4/N) 
So, on the surface - the districts that stayed masked longer did better. But... (5/N) 
I know what you're thinking. This isn't a randomized trial. Those districts that stayed masked are likely different than the ones that didn't. You're right! But maybe not in the way you think. (6/N)
The "stay masked districts" had kids with lower SES, more minority kids, and (critically) had larger classroom sizes. All these factors would suggest a *higher* COVID rate in those districts. (7/N) 
In other words, the confounding is in the wrong direction - if anything it makes these results stronger than reported. (8/N)
OK but maybe kids in the unmasked schools tested more to compensate? Classic ascertainment bias? Second author @EpiEllie clued me into this figure in the supplement. Kids in the masked schools got tested *more*. Again, this suggests true effects even larger than observed. (9/N) 
This is all pretty compelling data that masks averted some infections. Authors calculate ~12,000 extra infections in unmasked districts, roughly 1/3rd of total in the study period. (10/N)
Masks are annoying, but missing school is more annoying and these 12,000 extra kids would be out of school for a week+. Not to mention the (small but real) risk of more severe covid complications, long covid, MIS-C, etc. (11/N)
Is this a perfect study? Of course not. It's one state. It's observational. And no one study is ever definitive. Also, the magnitude of effect seen by delaying masks by 1 or 2 weeks seems larger than I would expect (but exponential growth is weird). (12/N)
I'm a realist - I get that we are not masking again right now. That's ok - but let's also be real about the benefits. Masking probably avoids absences. And kids need to be in school (in person) as much as possible. If (when?) cases tick up - we should remember that. (13/N).
I wrote in more detail about this study for @Medscape here: www.medscape.com/viewarticle/983270
And the primary article is here: www.nejm.org/doi/full/10.1056/NEJMoa2211029
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And the primary article is here: www.nejm.org/doi/full/10.1056/NEJMoa2211029
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