Epidemiological patterns and factors associated with mortality among diphtheria cases in Kano State, Nigeria (2022-2025): A retrospective cohort study

Introduction: Nigeria continues to report one of the highest burdens of diphtheria globally, with Kano State accounting for a disproportionate share of cases and deaths. Understanding epidemiological patterns and mortality determinants is essential for guiding targeted interventions. Objectives: To describe the distribution of diphtheria cases in Kano State by socio-demographic and clinical characteristics, Local Government Area, and month of reporting, and identify independent predictors of mortality. Methods: We conducted a retrospective cohort study of 21,369 patients with clinically compatible and laboratory-confirmed diphtheria using secondary data from medical records (2022-2025). Descriptive statistics summarized patient characteristics and disease distribution. Associations with mortality were assessed using chi-square or Fishers exact tests. Multivariable logistic regression identified predictors of mortality, reported as adjusted odds ratios (aORs) and 95% confidence intervals (CIs). Results: Of 21,369 patients, 13,557 (63%) were female, with a median age of 11 years (IQR: 7-13.6). Children aged 5-9 years accounted for 29% of cases. Prior contact with a case was reported in 2,633 (12%) patients, while 168 (1%) were laboratory-confirmed. Overall, 13,468 (63%) patients were unvaccinated and 5,381 (25%) fully vaccinated; Diphtheria antitoxin DAT was administered to 2,035 (10%). Cases occurred in all local government areas (LGAs), with four accounting for 60% of cases. Transmission occurred year-round, peaking in March 2024 and August 2023 and 2025. Overall, 1,131 deaths occurred (CFR: 5.3%). Compared with patients aged [≥]20 years, younger age groups had higher odds of mortality: 0-4 years (aOR = 26.74, 95% CI: 17.64-42.72), 5-9 years (aOR = 19.77, 95% CI: 13.17-31.33), 10-14 years (aOR = 8.21, 95% CI: 5.41-13.13), and 15-19 years (aOR = 2.46, 95% CI: 1.45-4.26). Prior contact with a case was associated with lower odds of mortality (aOR = 0.42, 95% CI: 0.30-0.55), whereas laboratory confirmation (aOR = 2.33, 95% CI: 1.48-3.56) and DAT receipt (aOR = 2.55, 95% CI: 2.19-2.96) were associated with higher odds. Prior vaccination was associated with lower odds of mortality among fully vaccinated (aOR = 0.52, 95% CI: 0.44-0.61) and partially vaccinated patients (aOR = 0.61, 95% CI: 0.47-0.80). Conclusions and recommendations: During the study period, diphtheria was more common among younger children, females, and the unvaccinated. Although concentrated in a few LGAs, transmission was widespread and varied seasonally. Mortality was associated with younger age, laboratory-confirmed infection, and use of diphtheria antitoxin, whereas vaccination and prior contact were protective. Reducing the burden requires sustained efforts to strengthen routine immunization, implement targeted vaccination campaigns, ensure early detection and prompt treatment, and improve contact tracing and case management.