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

Abstract

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 CO₂ 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). CO₂ 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 CO₂ 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 CO₂ 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.

Keywords: Air contamination; CO2 concentration; COVID-19 measures; SARS-CoV-2; Secondary schools.

Fig. 1

Study design and data-collection

Fig. 2

Dynamics of weekly SARS-CoV-2 incidence in schools and the Dutch general population between October 2020 and June 2021

Fig. 3

Weekly SARS-CoV-2 incidence per school for school-initiated COVID-19 measures.^*: A cohorting interventions B physical distancing C student displacement reductions D (hand) hygiene. * Cohorting interventions include separate walking routes, entrances, exits, start/stop moments class hours. Physical distancing includes fixed seating, extra breaks, and physical distancing (> 1.5 m). Student displacement reductions include dedicated classrooms for course hours and breaks. (Hand) hygiene includes use of splash guards, and access to disinfectant hand gel. For an overview of all school-initiated COVID-19 measures and calculation of the total score we refer to Supplement Table S1

Fig. 4

Weekly SARS-CoV-2 incidence per school by percent-time the difference between indoor and outdoor CO₂ concentration above threshold-level 400 ppm (Excellent), 550 ppm (Good), and 800 ppm (Acceptable). Percent-time of CO₂ concentrations above the threshold-level and weekly SARS-CoV-2 incidence per school. Crosses reflect individual weekly SARS-COV-2 incidence rates per school and dots reflect median values per school per period (pre-lockdown; post-lockdown). Percent-time of CO₂ concentrations above threshold was calculated by dividing the time the difference between indoor and outdoor CO₂ concentration exceeded 400 ppm, 550 ppm, and 800 ppm, respectively, by the total measurement time per occupied classroom. Measurements were averaged over classrooms to obtain a summary estimate per school

Fig. 5

Total bacterial load 16S qPCR response for EDC samples collected in school classrooms during pre-lockdown, lockdown and post-lockdown period, and those for EDC samples collected in rooms of nursing home patients in isolation. Total bacterial loads are depicted as 40 minus Ct-value thus higher values express higher levels. The study underlying the nursing home data from this figure is reported in Linde et al. 2022 [23]