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CDC Study Purporting To Find Substantial Protective Effects For School Mask Mandates Fails To Replicate

Last year, the CDC published a paper comparing Pediatric COVID-19 Cases in Counties With and Without School Mask Requirements. The authors looked at data from 520 United States counties, concluding that “Counties without school mask requirements experienced larger increases in … case rates … compared with counties that had school mask requirements.” Corona astrologers and face diaper fetishists everywhere have used the findings to argue for forcing healthy children who are at no risk to wear fasks masks for multiple hours each school day.

More county-level data on American infection rates and mask mandates has since become available, and two Toronto scientists have taken the opportunity to replicate the study, looking now at 1,832 counties. In a turn of events that will surprise nobody, they find that the larger dataset shows that mask mandates actually do zero, and that prior findings were almost surely an illusion.

Here are masked vs. unmasked case rates, using a smaller data pool similar to that from the CDC study:

Week 0 is the week of school reopening after the summer holidays.

Yes, the maskless counties seem to do worse! Yet the Toronto authors point out that the original CDC study only considered infection rates through the second week after schools reopened, which turned out to be “exactly the peak of school case numbers for [their] sample of counties.” This obscured the fact “that cases quickly declined in later weeks and did so faster in counties without mask mandates.” Even the smaller sample used by the CDC study, in other words, showed no difference in masked vs. unmasked counties by the six-week mark.

The replication, with a much bigger dataset, meanwhile, showed that maskless counties never led infections at all:

Note that, in the larger sample, the maskless start out with lower rates of infections and catch up; in the smaller sample, they started out with higher rates of infection and collapsed more quickly.

The authors note that the CDC study, by ending their analysis on 4 September 2021, effectively excluded counties with a school-start date after 14 August, which entailed an oversampling of southern states. I’ll fill in the blanks here: Counties in the American south tend to have fewer school mask requirements, and also to experience late summer infection spikes related to high temperatures and extensive reliance on climatisation.

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