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

• Published in: BMJ (Online) 2021-03, Vol.372 (Art. n616), p.o.A.-372:Art. n616<o.A.
• Main 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
• Format: Article
• Language: English
• Subjects: Adolescent; Child; Children; Cluster Analysis; Cohort analysis; Cohort Studies; Contact tracing; Coronaviruses; COVID-19; COVID-19 - epidemiology; COVID-19 Serological Testing; Disease transmission; Female; Fernunterricht; Humans; Infections; Longitudinal Studies; Male; Pandemics; Pandemie; Phlebotomy; Population; Prospective Studies; Regression analysis; SARS-CoV-2; Schule; Schulschließung; Schulunterricht; Schweiz; Seroconversion; Seroepidemiologic Studies; Serology; Severe acute respiratory syndrome coronavirus 2; Students - statistics & numerical data; Summer; Switzerland - epidemiology; Testen; Zürich
• ISSN: 1756-1833; 0959-8138; 1756-1833
• DOI: 10.1136/bmj.n616

AbstractObjectivesTo 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.DesignProspective cohort study.SettingSwitzerland 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.Participants275 classes in 55 schools; 2603 children participated in June-July 2020 and 2552 in October-November 2020 (age range 6-16 years).Main outcome measuresSerology of SARS-CoV-2 in June-July and October-November 2020, clustering of children who were seropositive within classes, and symptoms in children.ResultsIn 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. Clas