2022 Yes https://www.nature.com/articles/s41467-022-28170-6 Lasser, J. Europe Modeling Teachers Students Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Ventilation rates COVID rates In the data we find that 40% of all clusters involved no more than two cases, and 3% of the clusters only had more than 20 cases. The model shows that combinations of NPIs together with vaccinations are necessary to allow for a controlled opening of schools under sustained community transmission of the SARS-CoV-2 delta variant. For plausible vaccination rates, primary (secondary) schools require a combination of at least two (three) of the above NPIs.