Clustering and longitudinal change in SARS-CoV-2 seroprevalence in school-children: prospective cohort study of 55 schools in Switzerland
Authors: Ulyte Agne , Radtke Thomas , Abela Irene A. , Haile Sarah R. , Berger Christoph , Huber Michael , Schanz Merle , Schwarzmueller Magdalena , Trkola Alexandra , Fehr Jan , Puhan Milo A. , Kriemler Susi 
Published: December 2020

Background and aims

The facilitating role of schools in SARS-CoV-2 infection spread is still debated and the potential of school closures to mitigate transmission unclear. In autumn 2020, Switzerland experienced one of the highest second waves of the SARS-CoV-2 pandemic in Europe while keeping schools open, thus offering a high-exposure environment to study SARS-CoV-2 infections in schools. The aim of this study was to examine longitudinal change in SARS-CoV-2 seroprevalence in children and the evolution of clustering within classes and schools from June to November, 2020, in a prospective cohort study of school children in the canton of Zurich, Switzerland.

Methods

Children from randomly selected schools and classes, stratified by district, were invited to participate in serological testing of SARS-CoV-2 in June-July and October-November 2020. Parents of children filled questionnaires on sociodemographic and health-related questions. 55 schools and 275 classes within them were enrolled, with 2603 children participating in the first, and 2552 in the second testing (age range 6-16 years). We evaluated longitudinal changes of seroprevalence in districts and investigated clustering of seropositive cases within classes and schools.

Results

Overall SARS-CoV-2 seroprevalence was 2.4% (95% CrI 1.4%-3.6%) in summer and 4.5% (95% CrI 3.2%-6.0%) in not previously seropositive children in late autumn, leading to estimated 7.8% (95% CrI 6.2%-9.5%) of ever seropositive children, without significant differences among lower, middle and upper school levels. Among the 2223 children with serology tested twice, 28 (40%) of previously seropositive were negative, and 109 (5%) previously negative became seropositive. Seroprevalence was not different between school levels or sexes, but varied across districts (1.7% to 15.0%). Between June-July and October-November 2020, the ratio of diagnosed to newly seropositive children was 1 to 8. At least one newly seropositive child was detected in 47 of 55 schools and 90 of 275 classes. Among 130 classes with high participation rate, 0, 1-2 or ≥3 seropositive children were present in 73 (56%), 50 (38%) and 7 (5%) classes, respectively. Class level explained slightly more variation of individual serological results (standard deviation of random effects (SD) 0.97) than school level (SD 0.61) in the multilevel logistic regression models. Symptoms were reported for 22% of seronegative and 29% of newly seropositive children since summer.

Conclusions

Under a regimen of open schools with some preventive measures in place since August, clustering of seropositive cases occurred in very few classes and not across entire schools despite a clear increase in seropositive children during a period of high transmission of SARS-CoV-2.

Trial registration

ClinicalTrials.gov NCT04448717. https://clinicaltrials.gov/ct2/show/NCT04448717