The study proposes to conduct repeated cross-sectional seroprevalence studies testing for antibodies to SARS-CoV-2, including nucleocapsid (N, specific for infection, since only spike-encoding mRNA vaccines are currently approved for use in children), spike (S, indicating infection and/or vaccination), and receptor-binding domain (RBD, indicating neutralizing activity and protection from infection and/or vaccination) on a large number of children visiting Emergency Departments (ED) across Canada. This study aims to understand the current state of natural and vaccine-based COVID-19 immunity. The specific aims this project will address include: * Establish a structure that will allow ongoing use of residual blood samples to determine seroprevalence for infectious diseases and assess population-level exposure and immunity. * Determine the temporal association between the proportion of seropositivity to SARS-CoV-2 antigens (N, S, RBD) and public health measures by age and regions in Canada at baseline. * Evaluate the association between serologic measures and temporal trends of viral transmission, vaccination rates, and severe outcomes in children. The project’s primary objective is to determine, at baseline, the proportion of children with positive IgG against N and S by age and geographic region over time as an estimate of infection.