Diphtheria resurgence, drug-supply delays, and case fatality at a tertiary hospital in Adamawa State, north-eastern Nigeria: a retrospective cohort study (2023-2026)

Background Diphtheria caused by toxigenic Corynebacterium diphtheriae re-emerged in Nigeria from December 2022 as the country's largest ever recorded outbreak, accumulating over 20,000 suspected cases and 872 confirmed deaths by December 2025. The Borno-Adamawa-Yobe (BAY) states zone in north-eastern Nigeria is a recognised high-vulnerability cluster, yet no facility-level, longitudinal epidemiological data from Adamawa State have been published. Methodology/Principal findings We conducted a retrospective cohort study of all patients admitted with diphtheria to the Isolation Ward of Modibbo Adama University Teaching Hospital (MAUTH), Yola -- the sole federal university teaching hospital in Adamawa State -- from January 2023 to April 2026, using prospective admission register records. Sixty-one patients were identified (17.9% of 330 total isolation admissions). Admissions escalated 580% from 5 (2023) to 34 (2025). Median age was 8.0 years; 91.8% were under 15 years. The overall in-hospital case fatality rate (CFR) was 41.5% (22/53 known outcomes; 95% confidence interval: 29.0-55.0%). Annual CFR declined from 60.0% (2023) to 28.6% (2025), temporally consistent with improving diphtheria antitoxin access. A critical operational finding was the persistent shortage of intravenous erythromycin -- the mandated antibiotic for patients unable to swallow -- compelling oral administration in patients with pharyngeal pseudomembrane and dysphagia. Respiratory distress at presentation carried an 80% CFR; cardiac complication, 100%. Age and sex were not statistically significant mortality predictors. Gombi local government area contributed 16.4% of cases -- the highest burden among non-capital communities -- consistent with its role as a population movement corridor from Borno State's outbreak epicentre. A September-October seasonal peak (47.5% of admissions) was identified, diverging from the national January-April pattern. Conclusions/Significance This study provides the first peer-reviewed, facility-level diphtheria epidemiological dataset from Adamawa State. The in-hospital CFR substantially exceeds national surveillance averages due to referral bias and historical drug supply constraints. The declining CFR against rising admissions signals improving case management. Pre-positioning of diphtheria antitoxin and intravenous erythromycin before each August-October peak, accelerated childhood immunisation catch-up, and strengthened surveillance in Adamawa State are identified as urgent priorities.