2023 Yes https://bmjopen.bmj.com/content/bmjopen/13/7/e072284.full.pdf Falkenberg, Timo Europe Experimental Observational Students Elementary (K-5) Ventilation rates COVID rates COVID-19 community child health primary prevention public health. 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.