Study Design and population
This study has a cross-sectional design. We established a network of pediatricians who volunteered to take part in the study and focused on three areas/counties within Bavaria with very high, moderate and average infection rates as indicated by positive PCR tests per 100,000 inhabitants according to the Robert Koch Institute, the German center for disease prevention (figure 1). In areas where the number of willful study participants exceeded the capacity of local pediatricians, a study team supported sample collections. The sample collection took place in the three study areas (Tirschenreuth; Regensburg city and county; Oberbayern/ alpine region from May 22nd to July 22nd 2020 in a sliding window approach. Invitation to participate for children age 1 to 14 years was based on two approaches: (1) All children of that age group who were scheduled for a prevention program visit in 2020 with the respective pediatrician were invited to participate and (2) all children whose parents and the children themselves wanted to participate voluntarily were also tested. In those cases, also siblings older than 14 years were included into the study. The study was approved by the Ethics Committee of the University of Regensburg (file-number: 20-1865-101).