SARS-CoV-2 incidence in secondary schools; the role of national and school-initiated COVID-19 measures

L Jonker, K J Linde, A R de Boer*, E Ding, D Zhang, M L A de Hoog, S Herfst, D J J Heederik, P L A Fraaij, P M Bluyssen, I M Wouters, P C J L Bruijning-Verhagen

*Corresponding author for this work

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INTRODUCTION: Our aim was to gain insight into the effect of COVID-19 measures on SARS-CoV-2 incidence in secondary schools and the association with classroom CO 2 concentration and airborne contamination.

METHODS: Between October 2020-June 2021, 18 schools weekly reported SARS-CoV-2 incidence and completed surveys on school-initiated COVID-19 measures (e.g. improving hygiene or minimizing contacts). CO 2 was measured in occupied classrooms twice, and SARS-CoV-2 air contamination longitudinally using electrostatic dust collectors (EDC) and analyzed using RT-qPCR. National COVID-19 policy measures varied during pre-lockdown, lockdown and post-lockdown periods. During the entire study, schools were recommended to improve ventilation. SARS-CoV-2 incidence rate ratios (IRR) were estimated by Generalized Estimating Equation (GEE) models.

RESULTS: During 18 weeks follow-up (range: 10-22) SARS-CoV-2 school-incidence decreased during national lockdown (adjusted IRR: 0.41, 95%CI: 0.21-0.80) and post-lockdown (IRR: 0.60, 0.39-0.93) compared to pre-lockdown. School-initiated COVID-19 measures had no additional effect. Pre-lockdown, IRRs per 10% increase in time CO 2 exceeded 400, 550 and 800 ppm above outdoor level respectively, were 1.08 (1.00-1.16), 1.10 (1.02-1.19), and 1.08 (0.95-1.22). Post-lockdown, CO 2-concentrations were considerably lower and not associated with SARS-CoV-2 incidence. No SARS-CoV-2 RNA was detected in any of the EDC samples.

CONCLUSION: During a period with low SARS-CoV-2 population immunity and increased attention to ventilation, with CO 2 levels most of the time below acceptable thresholds, only the national policy during and post-lockdown of reduced class-occupancy, stringent quarantine, and contact testing reduced SARS-CoV-2 incidence in Dutch secondary schools. Widespread SARS-CoV-2 air contamination could not be demonstrated in schools under the prevailing conditions during the study.

Original languageEnglish
Article number1243
Number of pages12
JournalBMC Public Health
Issue number1
Publication statusPublished - 27 Jun 2023


  • Air contamination
  • CO  concentration
  • COVID-19 measures
  • SARS-CoV-2
  • Secondary schools


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