Epidemiological investigation and public health response to a diphtheria outbreak in Jalalabad Union, Sylhet, December 2023.

Objectives: Diphtheria incidence has significantly declined globally due to the Expanded Program on Immunization, yet breakthrough infections occur due to waning immunity in late childhood. Since 2010, Bangladesh has reported 148 cases. On December 10, 2023, a suspected diphtheria case was reported to the Institute of Epidemiology, Disease Control and Research, prompting an epidemiological investigation and public health response. Methods: Following World Health Organization guidelines, the team identified suspected and confirmed cases, conducted interviews with the index case and contacts, and collected samples for polymerase chain reaction testing. Risk communication and active case searches were carried out in the affected area. Results: The index case, a 10-year-old girl with classic diphtheria symptoms, was hospitalized for 15 days and recovered after receiving antibiotics and diphtheria antitoxin (DAT). Among 25 close contacts, 14 (56%) were suspected cases, and two (8%) were confirmed via polymerase chain reaction; these two positive cases were siblings of the index case and were previously vaccinated. Chemoprophylaxis, vaccination, isolation, and quarantine were implemented, although the two later confirmed cases did not receive DAT due to unavailability. Conclusions: Despite a high average coverage rate of 95% (2018-2022) for Penta 3, the outbreak revealed potential waning immunity or vaccine failure. The outbreak was effectively contained through timely confirmation, contact tracing, prophylactic antibiotics, immunization, and quarantine. Booster doses, quantification of antibody titer, and DAT access remain critical.