Long-term kinetics of anti-RBD IgG antibodies 16 months after COVID-19 vaccination in Morocco: a longitudinal cohort study

Understanding the long-term durability of vaccine-induced immunity is crucial for shaping public health strategy. This study aimed to evaluate the 16-month kinetics of anti-receptor-binding domain (RBD) IgG antibodies following COVID-19 vaccination in Morocco. In this longitudinal cohort study, 2,546 blood samples were collected from 1,273 participants across the Casablanca-Settat region at six and sixteen months after the last vaccine dose. Participants had received homologous or heterologous regimens with the BNT162b2, ChAdOx1‑S, or BBIBP‑CorV vaccines. Anti‑RBD IgG levels were quantified using a chemiluminescent microparticle immunoassay. Multivariable linear mixed‑effects modeling was used to evaluate associations with age, sex, vaccine platform, number of doses, prior infection, and comorbidity status. Antibody levels declined significantly between six and sixteen months among participants who received only two doses (β = -0.072, 95% CI - 0.088 to - 0.056; p < 0.001). Receipt of a third dose was independently associated with higher antibody levels (β = 0.180, 95% CI 0.070-0.290; p = 0.001), with no significant interaction between dose number and time (p = 0.717). Prior SARS‑CoV‑2 infection was associated with higher antibody levels in unadjusted analyses but was not independently associated with antibody magnitude after multivariable adjustment. Increasing age was associated with higher antibody levels (β = 0.008 per year; p < 0.001), and no significant interaction between age and time was observed (p = 0.338), indicating comparable antibody decay across age groups. A trend toward higher antibody levels in females compared with males was observed but did not reach conventional statistical significance (p = 0.052). Anti‑RBD IgG antibodies decline over time following primary vaccination but remain detectable at 16 months. Booster vaccination is independently associated with higher antibody levels. Antibody decay kinetics appear broadly comparable across subgroups based on age, sex, and dose number. These findings support continued booster vaccination strategies to sustain humoral immunity.