Trial Design
MAY
This RCT was conducted in 50 day care centers in the Oulu region of Finland and in the Department of Pediatrics and Adolescent Medicine, Oulu University Hospital, Finland, from August 1, 2020, to May 31, 2021. Participants were recruited between August 26 and November 25, 2020. Six-month follow-up was completed on May 31, 2021. Throughout the study period, during the COVID-19 pandemic, all day care centers and primary schools remained open in Finland.