Prevalence and incidence of antibodies against SARS-CoV-2 in children measured for one year in Belgium: a sero-epidemiological prospective cohort study - questionnaire, Merckx, Joanna, Vermeulen Melissa, Vandermeulen Corinne, Roelants Mathieu, Desombere I, and Duysburgh Els , 2020, Brussels, Belgium, (2020)