INTRODUCTION:
Despite the availability of effective vaccines, diphtheria continues to pose a threat in low-resource settings. In October 2024, a suspected diphtheria case was reported from District Mardan, Khyber Pakhtunkhwa (KP), Pakistan, triggering a field investigation. We aimed to assess the magnitude of the outbreak, identify gaps in immunization, and describe the public health response.

METHODS:
A cross-sectional outbreak investigation was conducted using WHO case definitions and standard EPI surveillance tools. Suspected cases were identified through active search and contact tracing. Nasopharyngeal specimens were collected and tested via quantitative PCR (qPCR). Descriptive statistics were used to summarize clinical features, demographic characteristics, and vaccination coverage.

RESULTS:
A total of 41 suspected cases were identified, with 3 confirmed via laboratory testing. The index case was a 12-year-old fully vaccinated female, clinically managed with antitoxin and antibiotics. Among the 44 children investigated, 34 (77%) had received all three pentavalent doses, while 3 (7%) were unvaccinated. Most (82%) were under five years of age. No complications or deaths occurred. Delays in case recognition and low community awareness were major challenges.

CONCLUSIONS:
This outbreak highlights the limitations of administrative vaccine coverage data and the need for robust community-based surveillance and catch-up immunisation strategies. The resurgence of diphtheria in vaccinated populations warrants urgent investment in booster dose programmes, stockpiling of antitoxin, and public health education in endemic settings.