Variation in SARS-CoV-2 seroprevalence in school-children across districts, schools and classes (2020)

Abstract Importance Understanding transmission and impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in school children is critical to implement appropriate mitigation measures. Objective To determine the variation in SARS-CoV-2 seroprevalence in school children across districts, schools, grades, and classes, and the relationship of SARS-CoV-2 seroprevalence with self-reported symptoms. Design Cross-sectional analysis of baseline measurements of a longitudinal cohort study (Ciao Corona) from June-July 2020. Setting 55 randomly selected schools and classes stratified by district in the canton of Zurich, Switzerland (1.5 million inhabitants). Participants Children, aged 6-16 years old, attending grades 1-2, 4-5 and 7-8. Exposure Exposure to circulating SARS-CoV-2 between February and June 2020 including public lock-down and school closure (March 16-May 10, 2020). Main Outcomes and Measures Variation in seroprevalence of SARS-CoV-2 in children across 12 cantonal districts, schools, and grades using a Luminex-based antibody test with four targets for each of IgG, IgA and IgM. Clustering of cases within classes. Analysis of associations of seropositivity and symptoms. Comparison of seroprevalence with a randomly selected adult population, based on Luminex-based IgG and IgA antibody test of Corona Immunitas. Results In total, 55 schools and 2585 children were recruited (1337 girls, median age 11, age range 6-16 years). Overall seroprevalence was 2.8 % (95% CI 1.6–4.1%), ranging from 1.0% to 4.5% across districts. Seroprevalence was 3.8% (1.9-6.1%) in grades 1-2, 2.5% (1.1-4.2%) in grades 4-5, and 1.5% (0.5-3.0%) in grades 7–8. At least one case was present in 36/55 tested schools and in 43/128 classes with ≥50% participation rate and ≥5 children tested. 73% of children reported COVID-19 compatible symptoms since January 2020, but none were reported more frequently in seropositive compared to seronegative children. Seroprevalence of children was very similar to seroprevalence of randomly selected adults in the same region in June-July 2020, measured with the same Corona Immunitas test, combining IgG and IgA (3.1%, 95% CI 1.4-5.4%, versus 3.3%, 95% CI 1.4-5.5%). Conclusions and Relevance Seroprevalence was inversely related to age and revealed a dark figure of around 90 when compared to 0.03% confirmed PCR+ cases in children in the same area by end of June. We did not find clustering of SARS-CoV-2 seropositive cases in schools so far, but the follow-up of this school-based study will shed more light on transmission within and outside schools. Trial registration http://ClinicalTrials.gov Identifier: NCT04448717 , registered June 26, 2020. https://clinicaltrials.gov/ct2/show/NCT04448717 Key Points Question What is the variation in seroprevalence of SARS-CoV-2 cases in school children across districts, schools and classes? Findings Among 55 randomly selected schools and 2585 children, 2.8 % (95% CI 1.6-4.1%) of children had SARS-CoV-2 antibodies, similar to the seroprevalence in the adult population in the same region but showing a much higher dark figure (89 versus 12 in adults). A third of tested classes had at least one SARS-CoV-2 case, and higher seroprevalence was observed in lower grades. Seropositive children did not report SARS-CoV-2 infection symptoms more often than seronegative children. Meaning The results so far do not suggest substantial transmission within schools. In contrast to the current literature, younger children seem to be infected slightly more often; the striking dark figure of 89 could be partly due to the fact that symptoms compatible with COVID - 19 were highly prevalent in children and do not help to differentiate between seropositive and seronegative children.