CDC Report: Schools without mask requirements were more likely to have Covid-19 outbreaks.

Association Between K–12 School Mask Policies and School-Associated COVID-19 Outbreaks — Maricopa and Pima Counties, Arizona, July–August 2021 – CDC Morbidity and Mortality Weekly Report

Commentary: Schools Without Mask Requirements More Likely to See COVID-19 Outbreaks – U.S. News

Commentary on Twitter

New @CDCMMWR finds schools in two Arizona counties w/o a mask requirement were 3.5 times more likely to have a #COVID19 outbreak than schools requiring masks from the start of the school year. CDC recommends universal masking for all K–12 schools. More: https://t.co/suuuLNItxJ. pic.twitter.com/zYYqXoLH3j

— CDC (@CDCgov) September 24, 2021