Objectives: In Kilifi, pentavalent coverage remains below the 90% target, with no reported diphtheria or tetanus cases and sporadic pertussis. However, absence of disease does not guarantee immunity. To characterize age-specific gaps and waning protection not captured by routine surveillance, we conducted serial seroprevalence studies of diphtheria, pertussis, and tetanus.
Methods: We analyzed randomly selected participants from multiple cross-sectional surveys within the Kilifi Health and Demographic Surveillance System. Immunoglobulin G antibodies were measured using a fluorescent bead-based multiplex immunoassay applying protective thresholds ≥0.011 IU/ml for diphtheria and tetanus. Pertussis antibodies were grouped by time since infection. Bayesian multilevel regression with post-stratification adjusted estimates for population structure and assay performance; associations with age 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 over time (τ = 0.68, P = 0.04). Tetanus protection was higher, with long-term seroprotection ranging from 10% to 39% and susceptibility <1%; trends were not significant. Older age was associated with lower seroprevalence. Among adults, <1% had long-term diphtheria seroprotection vs 36% for tetanus. Pertussis circulation was minimal, with 5% of children and <1% of adults, with antibody concentrations consistent with recent infection.
Conclusions: Although conventional serological thresholds suggest immunity gaps, particularly, for diphtheria, no diphtheria or tetanus outbreaks have occurred in Kilifi over the past decade. This indicates that antibody concentrations below standard thresholds may not equate to immediate susceptibility, but they do reflect a narrower margin of population immunity. Although this has not yet translated into disease, it could become relevant if transmission conditions change, underscoring the need to sustain high vaccination coverage and sensitive surveillance. Serology should, therefore, be viewed as a complementary tool, useful for tracking emerging vulnerability and informing future booster decisions if susceptibility increases.
Keywords: Diphtheria; Pertussis; Population immunity; Serological data; Seroprevalence; Tetanus.
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