Effect of portable HEPA filters on COVID-19 period prevalence: an observational quasi-interventional study in German kindergartens

OBJECTIVES: The aim of the study was to evaluate the effect of high-efficiency particulate air (HEPA) filters on COVID-19 period prevalence in kindergartens. DESIGN: The observational study follows an intervention design with the intervention group using HEPA filters and the control group not. SETTING: The study was conducted in 32 (10 intervention, 22 control) kindergartens (daycare centres) in Rhineland Palatinate (Germany). PARTICIPANTS: Data of 2360 children (663 intervention, 1697 control) were reported by the kindergarten heads. Data were collected on institutional level without any identifying information on individuals. Thus, all children of all facilities were included; however, no demographic data were recorded. INTERVENTIONS: The study followed a quasi-interventional design, as no formal intervention was conducted. A charity foundation equipped kindergartens with HEPA filters. These kindergartens were enrolled as intervention group. The control group was recruited from the neighbouring communities and districts. OUTCOME MEASURES: The primary outcome measure was the number of COVID-19 cases reported by the kindergarten heads, converted into period prevalence rates per 1000 population. RESULTS: The mean COVID-19 period prevalence rates of the control and intervention groups were 186 (95% CI: 137.8 to 238.9) and 372 (95% CI: 226.6 to 517.6) per 1000 children, respectively. The one-sided Wilcoxon rank-sum test indicates a p value of 0.989; thus, the hypothesised preventive effect of HEPA filters could not be confirmed in the kindergarten setting. CONCLUSIONS: While HEPA filters can significantly reduce the viral load in room air, this does not lead to reduced COVID-19 prevalence in the selected kindergartens in Germany. It is known that contagion mainly occurs via direct face-to-face air exchange during play and that the contaminated air does not necessarily pass through the filter prior to air exchange between children. The use of HEPA filters may also lead to a sense of security, leading to reduced preventive behaviour.