Abstract
Background: The maintenance of durable humoral immunity against diphtheria, tetanus, and Bordetella pertussis during adolescence is critical for long-term protection and epidemiological stability. However, the immunological impact of booster vaccination at 14 years, particularly for pertussis, remains incompletely understood. Methods: In this prospective, open-label study, 121 healthy 14-year-old adolescents received booster vaccination with Td-M (n = 34), Tdap (n = 51), or Tdap-M (n = 36). Serum IgG antibodies against diphtheria and tetanus toxoids and B. pertussis were quantified by ELISA before and one month after vaccination. Humoral responses were assessed using geometric mean titers and established seroprotection thresholds. Results: At baseline, all participants exhibited protective immunity to tetanus, while a minority lacked protective antibody levels against diphtheria (1.7%) and pertussis (6.9%). Booster vaccination elicited a robust increase in anti-diphtheria IgG across all groups, with no significant differences between vaccine formulations. All adolescents achieved high levels of tetanus-specific antibodies, indicating strong anamnestic responses, with the highest titers observed in the Td-M group. In contrast, pertussis-specific IgG responses were heterogeneous and showed no overall increase following revaccination, suggesting limited booster-induced humoral expansion in previously primed individuals. Notably, Tdap-M maintained more stable antibody levels compared to Tdap. All vaccines demonstrated good tolerability with only mild adverse events. Conclusion: Adolescent booster vaccination effectively reinforces humoral immunity against diphtheria and tetanus, reflecting preserved immunological memory. However, the absence of a significant pertussis-specific IgG boost suggests that pre-existing immunity, potentially shaped by prior vaccination and natural exposure, may limit the measurable serological response. These findings question the immunological necessity of routine pertussis revaccination at 14 years and highlight the need for optimization of booster strategies.