Abstract
Background: The lack of precise estimates on transmission risk hampers rational decisions on closure of educational institutions during the COVID-19 pandemic. Methods: Secondary attack rates (SARs) for schools and day-care centres were calculated using data from state-wide mandatory notification of SARS-CoV-2 index cases in educational institutions and additional information on routine contact tracing and PCR-testing. Findings: From August to December 2020, every sixth of overall 784 reported index cases caused a transmission in educational institutions (risk 0·17, 95% CI 0·14–0·19). In a subgroup, monitoring of 14,594 institutional high-risk contacts (89% PCR-tested) of 441 index cases revealed 196 secondary cases (SAR 1·34%, 1·16–1·54). Transmission was more likely from teachers than from students/children (incidence risk ratio [IRR] 3·17, 1·79–5·59), and from index cases in day-care centres (IRR 3·23, 1·76–5·91) than from those in secondary schools. In 748 index cases, teachers caused four times more secondary cases than children (1·08 vs. 0·25 secondary cases per index, IRR 4·39, 2·67–7·21). This difference was mainly due to common transmission from teacher index cases to teacher contacts (mean secondary cases 0·56 teacher to teacher vs. 0·04 child to teacher, IRR 13·3, 6·6–26·.7). Interpretation: In educational institutions, the risk of contacts to a confirmed COVID-19 case for infection is one percent, but varies depending on type of institution and index case. Hygiene measures targeting the day-care setting and teacher-to-teacher transmission are priorities in reducing the burden of infection and may promote on-site education during the pandemic. Funding: No particular funding was received for this study.Declaration of Interests: All authors, no conflicts.Ethics Approval Statement: The collection, analysis and communication of the presented data take place in response to the global COVID-19 pandemic and are mandated by the German Infectious Diseases Protection Act. Neither individual informed consent nor approval by an institutional review board are deemed necessary.
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