Seroprevalence of antibodies against Diphtheria, Tetanus and Pertussis over a 12-year period in children in Kilifi, Kenya (2009-2021)

Background: In Kilifi, no diphtheria or tetanus cases and only sporadic pertussis cases have been reported but the absence of disease does not guarantee sustained immunity. Case surveillance cannot reveal age-specific gaps or waning protection. We conducted serial seroprevalence studies to estimate population immunity trends for diphtheria, pertussis and tetanus. Methods: We analysed randomly selected participants from cross-sectional surveys within the Kilifi Health and Demographic Surveillance System (KHDSS). IgG antibodies were measured using a fluorescent bead-based multiplex immunoassay applying standard protective thresholds ≥0.011 IU/mL for diphtheria and tetanus. Pertussis antibodies were grouped by likely time since infection. Bayesian multilevel regression with post-stratification was used to adjust estimates for population structure and assay performance. Associations with age, sex and year were assessed using logistic regression. Results: Diphtheria seroprotection was low; only 5% of children had long-term seroprotection, with full protection ranging from 11% to 34% and minimal seroprotection from 40% to 52%. Minimal seroprotection increased significantly over time (τ=0.68, p=0.04). Tetanus protection was consistently higher, with long-term seroprotection ranging from 10% to 39% and susceptibility <1% in all years; trends were not significant. Older age was associated with lower seroprevalence for both diphtheria and tetanus. Among adults <1% had long-term diphtheria seroprotection versus 36% for tetanus. Pertussis circulation was minimal with 5% of children and <1% of adults showing antibody concentrations consistent with recent infection. Conclusion: Despite measurable immunity gaps, particularly for diphtheria, no outbreaks of diphtheria or tetanus have been observed in Kilifi and tetanus protection remains high. These findings suggest that the current immunisation programme continues to provide effective protection. Serology nonetheless highlights vulnerable cohorts and provides a baseline for monitoring susceptibility over time. Sustaining high coverage and maintaining sensitive surveillance will be essential to detect any future risk early and guide decisions on booster doses if susceptibility increases.