Clustering and longitudinal change in SARS-CoV-2 seroprevalence in school children in the canton of Zurich, Switzerland: prospective cohort study of 55 schools

Abstract

Objectives: To examine longitudinal changes in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence and to determine the clustering of children who were seropositive within school classes in the canton of Zurich, Switzerland from June to November 2020.

Design: Prospective cohort study.

Setting: Switzerland had one of the highest second waves of the SARS-CoV-2 pandemic in Europe in autumn 2020. Keeping schools open provided a moderate to high exposure environment to study SARS-CoV-2 infections. Children from randomly selected schools and classes, stratified by district, were invited for serological testing of SARS-CoV-2. Parents completed questionnaires on sociodemographic and health related questions.

Participants: 275 classes in 55 schools; 2603 children participated in June-July 2020 and 2552 in October-November 2020 (age range 6-16 years).

Main outcome measures: Serology of SARS-CoV-2 in June-July and October-November 2020, clustering of children who were seropositive within classes, and symptoms in children.

Results: In June-July, 74 of 2496 children with serological results were seropositive; in October-November, the number had increased to 173 of 2503. Overall SARS-CoV-2 seroprevalence was 2.4% (95% credible interval 1.4% to 3.6%) in the summer and 4.5% (3.2% to 6.0%) in late autumn in children who were not previously seropositive, leading to an estimated 7.8% (6.2% to 9.5%) of children who were ever seropositive. Seroprevalence varied across districts (in the autumn, 1.7-15.0%). No significant differences were found among lower, middle, and upper school levels (children aged 6-9 years, 9-13 years, and 12-16 years, respectively). Among the 2223 children who had serology tests at both testing rounds, 28/70 (40%) who were previously seropositive became seronegative, and 109/2153 (5%) who were previously seronegative became seropositive. Symptoms were reported for 22% of children who were seronegative and 29% of children who were newly seropositive since the summer. Between July and November 2020, the ratio of children diagnosed with SARS-CoV-2 infection to those who were seropositive was 1 to 8. At least one child who was newly seropositive was detected in 47 of 55 schools and in 90 of 275 classes. Among 130 classes with a high participation rate, no children who were seropositive were found in 73 (56%) classes, one or two children were seropositive in 50 (38%) classes, and at least three children were seropositive in 7 (5%) classes. Class level explained 24% and school level 8% of variance in seropositivity in the multilevel logistic regression models.

Conclusions: With schools open since August 2020 and some preventive measures in place, clustering of children who were seropositive occurred in only a few classes despite an increase in overall seroprevalence during a period of moderate to high transmission of SARS-CoV-2 in the community. Uncertainty remains as to whether these findings will change with the new variants of SARS-CoV-2 and dynamic levels of community transmission.

Trial registration: NCT04448717.

Conflict of interest statement

Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: support from Swiss School of Public Health (SSPH+), Swiss Federal Office of Public Health, private funders, funds of the cantons of Switzerland (Vaud, Zurich, and Basel), institutional funds of universities, and University of Zurich Foundation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

© Author(s) (or their employer(s)) 2019. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.

Figures

Fig 1

Flowchart of study participants. T1—testing in June-July 2020; T2—testing in October-November 2020. *Some classes were split or rearranged into multiple classes after summer break. †Eighteen of these children were enrolled from late August to early September 2020 (12 serological results, 18 questionnaires)

Fig 2

Seroprevalence estimates in children in June-July 2020 (T1), among children who were newly seropositive in October-November (T2), and those who were ever seropositive by October-November (T1+T2). Overall and school level specific estimates (lower school level: grades 1 and 2, children aged 6-9 years; middle school level: grades 4 and 5, children aged 9-13 years; upper school level: grades 7 and 8, children aged 12-16 years), and district level specific estimates for canton of Zurich. Districts are ranked in order of decreasing population size

Fig 3

Symptoms reported between July and November 2020 in children who were seronegative and newly seropositive (at autumn testing—T2)

Fig 4

Distribution of children who were seropositive at T1 and newly seropositive at T2 in classes with at least five children and 50% or more of children tested (29 classes in lower school level, children aged 6-9 years; 46 classes in middle school level, children aged 9-13 years; and 54 classes in upper school level, children aged 12-16 years). The summary figure depicts the serological status of children in autumn 2020 (T2)