Outcome	Number of studies	Overview of effect by study	Comparison used in each study	Effect direction per study (positive ▲; negative ▼; no change/mixed effects/conflicting findings ◀▶)
Outcome category: transmission‐related outcomes
Number or proportion of cases	2 observational/experimental studies (Isphording 2020; Vlachos 2020)	Isphording 2020: compared to school closures, three weeks after school openings, cases per 100,000 people decreased by 0.55 or 27% of a SD within the experimental group where co‐interventions included mask wearing, hand‐hygiene policy, respiratory etiquette, general physical distancing policy, modification of activities and exemption of high‐risk students. These were implemented alongside testing and quarantine and cohorting measures. The effect is strongest in the youngest age group of 0 to 14‐year‐old cases where the end of summer break is associated with a significant reduction in cases per 100,000 population of about 1.4 cases after 3 weeks for individuals up to 14 years (42% of a SD). Reductions for older age groups are smaller and insignificant: 0.82 cases in the group of 15 to 34 years (21% of a SD) and 0.43 cases in the group of 35 to 59 years (16% of a SD). The more vulnerable population of 60+ years appears to be unaffected by school openings. Levels of community transmission were relatively low at the time point at which schools were reopened, while compliance and agreement with social distance measures decreased strongly. It was not possible to disaggregate the effect of co‐interventions.	School closure	Positive ▲
		Vlachos 2020: among parents, exposure to open rather than closed schools resulted in a small increase in PCR‐confirmed infections (odds ratio (OR) 1.17, 95% CI 1.03 to 1.32]. Among lower secondary teachers the infection rate doubled relative to upper secondary teachers (OR 2.01, 95% CI 1.52 to 2.67). This spilled over to the partners of lower secondary teachers who had a higher infection rate than their upper secondary counterparts (OR 1.29, 95% CI 1.00 to 1.67). When analysing COVID‐19 diagnoses from healthcare visits and the incidence of severe health outcomes, results are similar for teachers but weaker for parents and teachers’ partners. The results for parents indicate that keeping lower secondary schools open had minor consequences for the overall transmission of SARS‐CoV‐2 in society. The results for teachers suggest that measures to protect teachers could be considered.	School closure	Negative ▼