Monday, August 15, 2022

School districts which removed mask policies had an immediate increase in COVID cases in both students and staff. This increase in cases got bigger over the next 10 weeks.

Dr Ellie Murray, ScD Profile picture
1h  13 tweets  4 min read
It's almost Back to School time & we're still seeing about 100k cases a day here in the US.

How do we stop COVID from going back to school too?

Our new pre-print confirms face masks must be part of our plan!

Let me explain /🧵

medrxiv.org/cgi/content/sh…
"What did we study?"

Halfway through the 2021/22 school year, some Massachusetts school districts dropped their mask requirements. This created a natural experiment.

We estimated: The causal effect of removing mask mandates in those districts on COVID levels 
"What did we find?"

School districts which removed mask policies had an immediate increase in COVID cases in both students and staff.

This increase in cases got bigger over the next 10 weeks.Figure 3 from the linked pr...
"How much did removing mask policies matter?"

Schools which removed mask policies had an average *increase* of 44.9 cases per 1,000 people over a 15 week period compared to if they had not removed their mask polices.

This adds up to an estimated 11,901 *extra* COVID cases! 
Implication: No mask requirement means kids miss school!

We estimated that students had an average increase of 39.9 cases per 1,000.

If all students who tested positive followed state guidelines to isolate for 5+ days, that's almost 20,000 *extra* missed school days! 
Implication: No mask requirement means teachers & school staff miss work!

For all staff, we an estimated 81.7 extra cases per 1,000!!

That's how many *more* teachers & staff got sick with COVID than would have if those school districts had kept mask requirements in place. 
"What about alternate explanations?"

School districts that dropped mask requirements tended to have better budgets, better buildings, better vaccination rates, and lower community COVID levels.

Our study shows those weren't enough to make dropping mask mandates work! 
"Why should you believe our study?"

This study is not a randomized controlled trial, it's an observational study, but it *can* still tell us about cause and effect.

For details on why & how the approach we used works, see this earlier thread:
Bottom line:

Eastern Massachusetts school districts which removed mask requirements in March 2022 saw a *dramatic increase* in COVID cases compared to if those districts had retained mask requirements.

And this was *despite* all other mitigation efforts those districts did. 
A huge thank you to the team at @HealthyBoston who worked so hard on this study, including lead author Dr Tori Cowger & senior author @KTHallPhDMPH, plus the rest of the amazing team: @OjikutuBisola, Sarimer Sanchez, Jaylen Clarke, @DrNataliaLinos, & @DrMaryTBassett
Adding to say: First author Dr Tori Cowger’s twitter is @ToriCowger (which I probably should have guessed😆🤦🏼‍♀️) 
One last thank you: to #econtwitter who do an excellent job of summarizing & tracking advancements in methods, and helped me stay up to date on new & improved DiD estimators over the past year. 

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https://threadreaderapp.com/thread/1557399491438952449.html

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