Originally, the study examined children in Bavaria for early markers of type 1 diabetes using blood samples. With the onset of the corona crisis in 2020, the scientists expanded the study to include antibodies against Sars-CoV-2. Even then, six times more children throughout Bavaria were infected with the virus than had been detected via PCR tests.
News
DZD Scientist at MDR 'aktuell' About Children and Corona

Dr. Markus Hippich. Source: Mona Walter; HMGU-IDF