SARS-CoV-2 Incidence in Secondary Schools; the Role of National and School-Initiated COVID-19 Measures and Indoor Air Quality

Abstract

Background: The aim of this study is to gain insight into the role of national and school-initiated COVID-19 measures, CO₂ concentration, and airborne contamination on the incidence of SARS-CoV-2 in secondary schools.

Methods: Between October 2020 - June 2021 participating secondary schools weekly reported their SARS-CoV-2 incidence for at least 10 consecutive weeks. Follow-up furthermore included questionnaires on school-initiated COVID-19 measures (e.g. improving hygiene or minimizing student-student and student-staff contacts), CO₂ measurements in occupied classrooms at two time-points (as proxy for ventilation), and repeated SARS-CoV-2 air contamination measurements using electrostatic dust collectors (EDC). Based on national COVID-19 policy three periods were defined: Until December 2020 (pre-lockdown), schools operated at full occupancy with limited testing and quarantine rules, followed by a national lockdown with nearly complete school closure until February 2021. Post-lockdown schools operated at reduced class-occupancy (50%) and expanded access to SARS-CoV-2 testing for contacts and quarantine rules. Generalized Estimating Equation (GEE)-based negative binomial models were used to estimate incidence rate ratios (IRR) of the effect of national policy, school-initiated COVID-19 measures, CO₂ concentration, SARS-CoV-2 air contamination, and on SARS-CoV-2 school-incidence. Models were adjusted for background municipal SARS-CoV-2 population incidence and school type (theoretical vs vocational training). EDC samples were analyzed for the presence of SARS-CoV-2 RNA by RT-qPCR.

Findings: In total, 18 secondary schools had a median follow-up of 18 weeks (range: 10-22). Weekly 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, 95% CI: 0.39-0.93) compared to pre-lockdown. No additional effect of school-initiated COVID-19 measures was found. Pre-lockdown, the IRRs for percent-time the CO₂ concentration exceeding the acceptable threshold of 800, 550 and 400 ppm above outdoor level was 1.08 (95%CI: 1.00-1.16; 1.10 (1.02-1.19), and 1.08 (0.95-1.22), respectively. Post-lockdown, CO₂ concentrations were generally lower and no association was observed. No SARS-CoV-2 RNA was detected in any of the samples from longitudinal air monitoring in schools.

Interpretation: During a period with low population immunity to SARS-CoV-2, a national COVID-19 policy including reduced class-occupancy, expanded quarantine and testing of contacts reduced the incidence of SARS-CoV-2 in Dutch secondary schools. Additional school-initiated COVID-19 measures did not reduce SARS-CoV-2 incidence. A consistent effect of reduced ventilation could not be confirmed in our study and effects may depend on other classroom conditions. Widespread SARS-CoV-2 air contamination was uncommon in schools, suggesting transmission occurred mostly by close range contact under the prevailing school conditions at the time.

Funding: ZonMW COVID-program.

Declaration of Interest: The authors have no conflict of interests to declare.

Ethical Approval: The study was conducted according to the guidelines of the declaration of Helsinki. Medical Research Ethics Committee NedMec waived EC approval (ref:20-621/C) as this study was not subjected to the Medical Research Involving Human Subjects Act (WMO).

Note: This paper has been published by BMC Public Health, 27 June 2023, https://bmcpublichealth.biomedcentral.com/articles/10.1186/s12889-023-16146-0)

Keywords: SARS-CoV-2, COVID-19 measures, secondary schools, air contamination, ventilation