The aim of the study was to assess the seroprevalence of SARS-CoV-2 IgG antibodies in children hospitalized for reasons other than COVID-19. The study was conducted among 686 children, aged 2 weeks to 18 years, hospitalized in the Department of Paediatrics, Infectious Diseases, and Hepatology in Bydgoszcz, Poland, in the period from 1 June 2021 to 30 April 2022. The presence of anti-SARS-CoV-2 antibodies was detected in 392 (57%) children. Since December 2021, a significantly larger proportion of children with a positive titer of anti-SARS-CoV-2 antibodies has been observed, constituting as much as 87.5% of patients hospitalized in April 2022. In total, 69% of children with detected anti-SARS-CoV-2 antibodies were children under 5 years of age. The highest mean amounts of titers of anti-SARS-CoV-2 antibodies were observed in children over 10 years of age. The conducted studies showed the presence of anti-SARS-CoV-2 antibodies in a significant group of hospitalized children and an increase in the percentage of these children during the fourth and fifth wave of COVID-19 in Poland caused by the Delta and Omicron variants, respectively. The vast majority of parents of the studied children had no knowledge of the COVID-19 infection in their charges, which may indicate asymptomatic infection or mild course of the disease.
Keywords: COVID-19; SARS-CoV-2; antibody; children.