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 (median age 7.31 months, IQR 2.9-31.7), 67 (54%) tested positivefor SARS-CoV-2. Positive cases had higher serum anti-Spike IgA and IgG at inclusion and T=3 months (p<0.01 and p=0.021 respectively), but this difference became non-significant after 12 months. Antibody levels remained stable between 12 and 24 months in both groups. Mucosal antibody responses followed a similar pattern. The correlation between anti-N and anti-S IgG mucosal and serum levels was strong (0.61-0.76, p<0.01), while IgA only showed moderate correlations (0.28-0.64, p-value <0.01). In Dutch children, one month post-infection, higher levels of serum anti-S (p=0.018) and anti-N (p<0.01) IgG were observed compared to South African age-matched counterparts at T=3 months. Conclusion: Hospitalised South African children exhibit a diminished SARS-CoV-2 antibody response compared to Dutch children with mild infections, possibly linking reduced antibodies to more severe disease. Sustained antibody levels in South African children ...