Background: Children represent a small fraction of global COVID-19 cases, but data from low- and middle-income countries suggest that children in these regions may face high morbidity and mortality rates. Objectives: To assess the longitudinal dynamics of SARS-CoV-2 antibodies in children presenting with respiratory tract infections (RTIs), compare antibody responses between South African and Dutch pediatric cohorts, and investigate the correlation between mucosal and systemic antibody levels. Methods: Between June 2020 and September 2022, 124 children hospitalised with RTIs at Tygerberg Hospital (Cape Town, South Africa) were enrolled. Serum and saliva for Luminex multiplex immunoassay was collected at baseline, and 3, 12, 18, and 24 months thereafter. A Dutch cohort of 16 children with asymptomatic/mild COVID-19 infection not requiring admission was included in March-April 2020. Statistical analyses included Wilcoxon signed-rank tests and Spearman correlations. Results:Of the 124 South African children ...