2024 Yes Effect of portable HEPA filters on COVID-19 period prevalence: an observational quasi-interventional study in German kindergartens https://bmjopen.bmj.com/content/bmjopen/13/7/e072284.full.pdf Falkenberg, Timo Europe Experimental Observational Students Elementary (K-5) Ventilation rates Filtration COVID-19 COVID-19 community child health primary prevention public health. An observational study in German kindergartens found no significant difference in COVID-19 prevalence between facilities with or without HEPA filters. Findings suggest filters alone may not prevent transmission due to direct child-to-child exposure. 2023