Community burden of diphtheria during the 2023-24 epidemic in Kano State, Nigeria: a population-based household survey

Background Nigeria has experienced its largest recorded diphtheria outbreak since late 2022, centred on Kano State, where facility-based surveillance documented over 25,000 confirmed cases. The true community burden remains unknown. We conducted a population-based household survey to estimate community attack rates, mortality, vaccination coverage, and determinants of infection and death. Methods We performed a retrospective household survey (September-October 2024) using spatially randomised cluster sampling (65 clusters, ~15 households each; recall period January 2023 to interview). Survey-weighted analyses, multivariable logistic regression, and sensitivity analyses were used. Findings We enrolled 7,998 individuals from 1,068 households. The community attack rate was 1.1% (95% CI 0.7-1.4), 4.2 times (2.7-5.3) higher than facility-based estimates. The case fatality ratio was 8.8% (1.9-15.6) overall and 21.3% among children under five; two thirds of deaths occurred at home. Delayed care-seeking of four or more days was associated with markedly higher mortality (risk ratio 32.6, 95% CI 2.4-450.0). Vaccination was strongly protective against death (vaccine effectiveness 57%, 95% CI 34- 72%; E-value 4.07). Among campaign-eligible children, routine EPI coverage was 56.6%; the reactive campaign reached few previously unvaccinated children (99.7% overlap with prior recipients), leaving 11.6% of eligible children unvaccinated. Interpretation Community diphtheria burden substantially exceeded facility surveillance estimates, with most deaths occurring outside the health system. Delayed care-seeking and low vaccination coverage were the main drivers of mortality, highlighting the need for improved community surveillance, decentralised care, and better-targeted vaccination.