Adult Seroprotection Gaps Against Diphtheria and Tetanus in Urban China: Repeated Cross-Sectional Serosurveillance in Pudong, Shanghai, 2017–2025

Background: Adult susceptibility to diphtheria and tetanus may increase as vaccine-induced immunity wanes, yet repeated population-based serosurveillance data in China are limited. Methods: We analyzed annual serosurveys conducted in Pudong New Area, Shanghai, China, from 2017 to 2025 among healthy adults aged 20–49 years. Diphtheria and tetanus IgG concentrations were measured by ELISA. Seroprotection was defined as antibody concentration ≥0.1 IU/mL. Antibody concentrations were further categorized as <0.01, 0.01–<0.1, 0.1–<1.0, and ≥1.0 IU/mL, and geometric mean concentrations (GMCs) were calculated. Multivariable logistic regression models were fitted to assess factors associated with non-protection, including survey year, age group, and household registration. Sensitivity analyses excluding the 2018 survey year were conducted. Results: A total of 2376 serum samples were included. Overall seroprotection was 21.46% for diphtheria and 13.80% for tetanus. The proportion protected against both antigens was 9.05%, while 73.78% showed concurrent non-protection against both antigens. The overall GMC was 0.032 IU/mL (95% CI: 0.030–0.034) for diphtheria and 0.018 IU/mL (95% CI: 0.017–0.019) for tetanus. Concentrations ≥1.0 IU/mL were uncommon for both antigens. Adults aged 40–49 years had higher odds of non-protection than those aged 20–29 years for diphtheria (OR: 2.43, 95% CI: 1.85–3.21) and tetanus (OR: 2.94, 95% CI: 2.11–4.13). Non-local residents also had higher odds of non-protection than local residents for diphtheria (OR: 1.55, 95% CI: 1.24–1.93) and tetanus (OR: 2.81, 95% CI: 2.15–3.69). Seroprotection varied across survey years, with a marked nadir in 2018. Sensitivity analyses excluding 2018 attenuated most year-specific associations, whereas age- and residence-related differences persisted. Conclusions: Healthy adults aged 20–49 years in Pudong showed low seroprotection and low GMCs against both diphtheria and tetanus, with a high proportion concurrently non-protected against both antigens. These findings highlight a persistent adult immunity gap and support further evaluation of adult booster strategies and enhanced serosurveillance.