Population waning trajectories of vaccine-induced tetanus immunity in Zhejiang, China

Objective: Tetanus that occurs outside the neonatal period is not a statutory reporting disease in many regions, and few countries have comprehensive surveillance systems. To assess the level and durability of vaccine-induced tetanus immunity in a healthy population in Zhejiang, where long-term population-based data on non-neonatal tetanus immunity remain limited. Methods: A cross-sectional study was conducted in 2024 among 3,659 healthy individuals. Anti-tetanus toxoid IgG concentrations were measured by ELISA and expressed in IU/mL. Population immunity was evaluated by geometric mean concentration (GMC), seropositivity (≥0.01 IU/mL), and seroprotection (≥0.1 IU/mL). Antibody persistence was estimated according to age- and dose-specific antibody profiles. Generalized additive models (GAMs) with smoothing functions for the time since last doses were used to model nonlinear antibody trajectories overall, gender, and by dose group. Results: The overall Geometric Mean Concentration (GMC) of tetanus IgG antibodies was 0.064 IU/mL, with seropositivity (≥0.01 IU/mL) and seroprotection (≥0.1 IU/mL) rates of 85.05% and 52.94%, respectively. GMC peaked at 0.175 IU/mL in children aged 6-11 months, remained relatively high through 2 years of age, and then generally declined to 0.008 IU/mL in adults aged 40-59 years. By dose, GMC increased from 0.013 IU/mL in the 0-dose group to 0.156 IU/mL in the 3-dose group, then declined after 4 and 5 doses. Seroprotection exceeded 89% after 2-3 doses. GAM-fitted trajectories showed a rapid post-vaccination peak followed by gradual waning, with antibody levels remaining above 0.1 IU/mL for 8.81 years overall. Waning was slower in men than in women (7.87 vs. 6.66 years), and the estimated duration of seroprotection was 6.03, 12.86, and 3.07 years after three, four, and five doses, respectively. Conclusions: A high seropositivity rate does not necessarily indicate durable seroprotection against tetanus. Booster immunization strategies should incorporate both vaccination history and time since the last dose to sustain long-term protection.