Swiss research shows clusters of SARS-CoV-2 infection are rare within school classes

Swiss research shows clusters of SARS-CoV-2 infection are rare within school classes


The role played by school children in transmitting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is a controversial issue in many countries. Although infection rates can be high in children, they rarely develop severe coronavirus disease 2019 (COVID-19) symptoms or health outcomes. Despite that, schools in many countries were shut down during the first wave of the pandemic to control the transmission of the virus, which resulted in disruption of education for over 1.5 billion students worldwide.

Analyzing longitudinal changes in SARS-CoV-2 seroprevalence and clustering in school children in Zurich during the first and second waves of the pandemic

Researchers from Switzerland recently presented the results of a longitudinal cohort study they conducted in the city of Zurich. The prospective cohort study’s key objective was to analyze the longitudinal changes in SARS-CoV-2 seroprevalence and assess the clustering of seropositive children in school classes from June to November 2020 in Zurich, Switzerland.

Switzerland is one of the countries most affected by the second wave of SARS-CoV-2 pandemic during autumn 2020 in Europe. Since the schools in Switzerland stayed open, they offered a moderate to high exposure environment to study SARS-CoV-2 infections. Children were randomly chosen from different schools and classes, stratified by district, and were invited for serological SARS-CoV-2 testing. Parents of the selected children completed questionnaires on health and sociodemographic-related questions.

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The participants of the study were from 275 classes in 55 schools. A total of 2,603 children participated in the study in June-July 2020, and 2,552 children participated in October-November 2020. The age range of the children was between 6 and 16 years. Primary outcomes measured included SARS-CoV-2 seroprevalence in June-July and October-November 2020, clustering of seropositive children within classes, and the presence of symptoms in children.

SARS-CoV-2 seroprevalence was 2.4% in the summer and 4.5% in late autumn

In June-July 2020, 74 children were seropositive out of 2496 children whose serological results were available. In October-November 2020, the number of seropositive children increased to 173 out of 2,503 children. Overall, SARS-CoV-2 seroprevalence in the summer was 2.4% and that in late autumn was 4.5% in children who were not seropositive earlier. This led to a total of 7.8% of seropositive children. Seroprevalence varied across different districts. In the autumn, it was between 1.7 and 15.0%.

No significant differences were observed among lower, middle, and upper school levels or among children aged 6-9 years, 9-13 years, and 12-16 years, respectively. Out of the 2,223 children who were tested both in the summer and in the autumn, 28/70 or 40% of previously seropositive children became seronegative, and 109/2,153 or 5% of previously seronegative children became seropositive. 22% of seronegative children and 29% of newly seropositive children since the summer showed symptoms. The ratio of children with SARS-CoV-2 infection to seropositive children was 1 to 8 between July and November 2020.

At least one child was detected in 47 out of 55 schools and in 90 out of 275 classes who was newly seropositive. Out of 130 classes with a high participation rate, no seropositive children were found in 73 or 56% of classes; 1 or 2 children were seropositive in 50 classes (38%), and at least 3 children were seropositive in 7 classes (5%). In the multilevel logistic regression models, school level explained 8% and class level explained 24% of seropositivity variance.

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Findings show that clusters of SARS-CoV-2 infection are rare within school classes

With schools in Switzerland open since August 2020 and some preventive strategies in place, clustering of seropositive children occurred in only a few classes despite a spike in overall seroprevalence during a period of moderate to high SARS-CoV-2 transmission in the community. Whether these findings will be different with the emergence of new SARS-CoV-2 variants and dynamic community transmission levels remains uncertain.

“Future testing rounds of this study will provide insights on transmission within classes over prolonged periods during dynamic levels of community transmission and the spread of new SARS-CoV-2 variants.”

Journal reference:

  • Ulyte A, Radtke T, Abela I A, Haile S R, Berger C, Huber M et al. Clustering and longitudinal change in SARS-CoV-2 seroprevalence in school children in the canton of Zurich, Switzerland: prospective cohort study of 55 schools BMJ 2021; 372 :n616 doi:10.1136/bmj.n616, https://www.bmj.com/content/372/bmj.n616
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