Day 1017: Yet Another Bad Mask Study

Of course, it's about masks. The NEJM paper is about an observational study of Boston schoolchildren, so it's been especially newsworthy here in Massachusetts. Vinay Prasad brought in Tracy Høeg to debunk the thing:

In the Boston study, the identified masking effect size against cases that is implausibly high. They say the dropping of the mask mandates corresponded with an additional 11,901 cases, which was 33.4% of ALL cases in the unmasked districts. Among the staff they found 40.4% of the cases to be attributable to the lifting of the mask mandates.

This is unrealistic considering most cases come from the community into the school AND we have a randomized study from Bangladesh failing to find any effect of either community or cloth masking in anyone under 50 (and that signal was modest, at around 11% decrease rate with surgical masks, which was uncertain, and no significant decrease with cloth masks). We also have a regression discontinuity design study from Spain which takes advantage of the fact that 5-year-olds don’t mask and 6 year old do and there was no significant discontinuity from age 5-6 as compared with other ages to suggest an effect of masking on case rates.

Additionally, the authors of the Boston study made the difficult-to-understand choice of “consider[ing] community rates of COVID-19 as part of the causal effect of school masking policies rather than a source of bias” in other words they saw community case rates to be a result of school masking policies/school case rates rather than community case rates to be the major source of school cases. A large body of research has suggested the opposite finding COVID-19 in schools is up to 10-20x more likely to come from outside of the school than from within. This includes a study from the UK where children <12 were not masked.

But they did plot the results of school COVID cases vs community covid cases and, as you can see here: [graph omitted] school case rates had a similar relationship to city/town case positivity rates in all districts, with all school case rates similar in relation to that of the community, which speaks against any large impact of school masking. Further, it is unclear why cases would be presumed to be coming from the school to the community with the school peak lagging the community peak in two of the districts or why any difference seen in the masked district would be presumed to be due to masks when the March 17 and the did not lift groups appear so similar in the difference between school (black) and community (orange) case rates. Looking overall at how much the mask district differs from the community compared with the unmasked districts, it’s clear the difference is modest---and of course may not be attributable to masks!

They both note the paper's strange focus on structural racism.