Evolution of SARS-CoV-2 seroprevalence and clusters in school children from June 2020 to April 2021 reflect community transmission: prospective cohort study Ciao Corona

  1. Agne Ulyte
  2. Thomas Radtke
  3. Irene A. Abela
  4. Sarah R. Haile
  5. Priska Ammann
  6. Christoph Berger
  7. Alexandra Trkola
  8. Jan Fehr
  9. Milo A. Puhan
  10. Susi Kriemler

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Abstract

Objectives

To longitudinally assess severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence and clustering of seropositive children within school classes in March-April 2021 compared to June-July and October-November 2020. To examine the evolution of symptoms and the extent of under-detection of SARS-CoV-2 in children.

Design

Prospective cohort study of randomly selected schools and classes.

Setting

Schools remained open for physical attendance in Switzerland from May 2020 to the end of 2020/2021 school year. Lower school level (age range 7-10 years) and middle school level (8-13 years) children in primary schools, and upper school level (12-17 years) children in secondary schools were invited for SARS-CoV-2 serological testing in the Ciao Corona study in the canton of Zurich, Switzerland. Three testing rounds were completed in June-July 2020 (T1; after the first wave of SARS-CoV-2 infections), October-November 2020 (T2; during the peak of the second wave), and March-April 2021 (T3; after the second wave and with SARS-CoV-2 variants of concern becoming dominant). Parents completed questionnaires on sociodemographic information and symptoms.

Participants

2487 children (median age 12 years, age range 7-17 years) recruited from 275 classes in 55 schools participated in the testing in March-April 2021; total of 2974 children participated in at least one of the 3 testing rounds.

Main outcome measures

SARS-CoV-2 serology results; clustering of seropositive children within classes; reported symptoms.

Results

The proportion of children who were SARS-CoV-2 seropositive increased from 1.5% (95% credible interval (CrI) 0.6% to 2.6%) in June-July 2020, to 6.6% (95% CrI 4.0% to 8.9%) in October-November, and to 16.4% (95% CrI 12.1% to 19.5%) in March-April 2021. By March-April 2021, children in upper school level (12.4%; 95% CrI 7.3% to 16.7%) were less likely to be seropositive than those in middle (19.5%; 95% CrI 14.2% to 24.4%) or lower school levels (16.0%; 95% CrI 11.0% to 20.4%). Children in the upper school level had a 5.1% (95% CI -9.4% to -0.7%) lower than expected seroprevalence by March-April 2021 than those in middle school level, based on difference-in-differences analysis. The ratio of PCR-diagnosed to all seropositive children changed from 1 to 21.7 (by June-July 2020) to 1 to 3.5 (by March-April 2021). Symptoms were reported by 37% of newly seropositive and 16% seronegative children. Potential clusters of 3 or more newly seropositive children were detected in 24 of 119 (20%) classes with a high participation rate, from which a median of 17 clusters could be expected due to random distribution of seropositive children within the classes. Clustering was lowest in middle and upper school levels. Retention rate in the cohort was high (84% of T1 participants attended T3). Among participants, supporting society and research were reported more commonly for participation than personal reasons. Fear of blood sampling was the most frequently reported reason for non-participation, reported for 64% of children.

Conclusions

By March-April 2021, 16.4% of children and adolescents were seropositive in the canton of Zurich, Switzerland. The majority of clusters of SARS-CoV-2 seropositive children in school classes could be explained by community rather than intra-class transmission of infections. Seroprevalence and clustering was lowest in upper school levels during all timepoints.

Trial registration

ClinicalTrials.gov NCT04448717 .

What is already known on the topic

  • Transmission of SARS-CoV-2 in school setting largely followed community transmission in 2020.

  • With implemented preventive measures, secondary attack rates were low and clustering of SARS-CoV-2 infections within classes and schools (outbreaks) were observed rarely.

What this study adds

  • With high community incidence and new variants of SARS-CoV-2, seroprevalence increased in school children between October 2020 – March 2021 in the canton of Zurich in Switzerland, and was higher in lower school levels.

  • Most of the potential clusters of children who tested seropositive within classes could be explained by community rather than intra-class transmission of SARS-CoV-2, especially in middle and upper school levels.

  • More children who tested seropositive in March-April 2021 were diagnosed and reported symptoms potentially related to SARS-CoV-2 infection more frequently than those who tested seropositive in June-July or October-November 2020.

  • The most frequent reason for non-participation was fear of blood sampling (62% of children).

Article activity feed

  1. ScreenIT

    SciScore for 10.1101/2021.07.19.21260644: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    NIH rigor criteria are not applicable to paper type.

    Table 2: Resources

    Antibodies
    SentencesResources
    The composite outcome allowed to estimate the proportion of all children who had SARS-CoV-2 antibodies (i.e., were infected) any time by a specific testing, rather than cross-sectional seroprevalence at a single testing, and was measured for T1, T2 and T3 separately (outcomes T1, T12, and T123).
    T1
    suggested: None
    T12
    suggested: None
    The Bayesian approach permitted adjusting for the sensitivity and specificity of the SARS-CoV-2 antibody test and the hierarchical structure of cohort (individual, school and district levels).
    SARS-CoV-2
    suggested: None

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: An explicit section about the limitations of the techniques employed in this study was not found. We encourage authors to address study limitations.

    Results from TrialIdentifier: We found the following clinical trial numbers in your paper:

    IdentifierStatusTitle
    NCT04448717RecruitingLongitudinal Study of Seroprevalence of SARS-CoV-2 Antibodie…

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    Results from scite Reference Check: We found no unreliable references.

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  2. Version published to 10.1101/2021.07.19.21260644 on medRxiv