Heterogeneous evolution of SARS-CoV-2 seroprevalence in school-age children: Results from the school-based cohort study Ciao Corona in November-December 2021 in the canton of Zurich

Abstract

Background Much remains unknown regarding the evolution of SARS-CoV-2 seroprevalence and variability in clustering of seropositive children in districts, schools, and classes as only a few school-based cohort studies exist. Vaccination of children, initiated at different times for different age groups, adds additional complexity to understand how seroprevalence developed in the school aged population. Aim We investigated the evolution of SARS-CoV-2 seroprevalence in children and clustering in districts, schools, and classes in Switzerland from June/July 2020 to Nov/Dec 2021. Methods SARS-CoV-2 antibodies were measured in primary and secondary school children from randomly selected schools in the canton of Zurich in Oct/Nov 2020, Mar/April, and Nov/Dec 2021. Variability of clustering in schools was expressed as maximum minus minimum seroprevalence in a class and summarized as median (interquartile range). Results 1854 children from 288 classes in 43 schools were tested, with median age 12 (range 6-17), 49% 12+ vaccinated. Seroprevalence increased from 5.6% (95% CrI: 3.5-7.6%) to 31.1% (27.0-36.1%) in unvaccinated children, and 46.4% (42.6-50.9%) in all children (including vaccinated). Earlier in the pandemic, seropositivity rates in primary schools were similar to those in secondary schools, but by late 2021, primary schools had 12.3% (44.3%) lower seroprevalence for unvaccinated (all) subjects. Variability in seroprevalence among districts and schools increased more than twofold over time, and in classes from 11% (7-17%) to 40% (22-49%). Conclusion Seroprevalence in children increased greatly, especially in 2021 following introduction of vaccines. Variability in seroprevalence was high and increased substantially over time, suggesting complex transmission chains.

Potential for bias in (sero)prevalence estimates when not accounting for test sensitivity and specificity

“…As an example of this, take the Ciao Corona study(12), a school-based longitudinal study of seroprevalence in Swiss school children with 5 rounds of SARS-CoV-2 antibody testing between June 2020 and June 2022, covering a range of seroprevalences in the population (Trial Registration: ClinicalTrials.gov NCT04448717). The study was approved by the Ethics Committee of the Canton of Zurich, Switzerland (2020-01336).…”