Hospital-Acquired Influenza Outbreak in an Internal Medicine Ward of One Hospital, Phitsanulok Province, September – October 2025

Abstract

On October 3, 2025, the epidemiology unit was notified of 10 cases of influenza detected in Medical Ward A. An outbreak investigation was conducted to confirm the diagnosis and outbreak, describe the epidemiological characteristics, and evaluate the investigation process. A descriptive study and active case finding were performed in Medical Ward A. A suspected case was defined as a person with at least two of the following symptoms: fever, cough, sore throat, or shortness of breath, with symptom onset occurring three days or more after admission. A confirmed case was a suspected case with laboratory detection of influenza virus. The investigation process was evaluated using the 7-1-7 framework of the Bureau of Epidemiology, Division of Disease Control. A total of 17 cases met the influenza case definition (attack rate 13.4%). Two cases were ward staff (attack rate 5.3%), and 15 cases were patients (attack rate 16.8%). Severe illness occurred in 29.4% of cases. Among these cases, influenza A (H1N1) was confirmed. The case fatality rate was 17.6%. The outbreak likely began with exposure to an infected family member and subsequently spread to other patients in the same area. The initial outbreak investigation and control measures were completed within 4 days. Hospitals should strengthen infection prevention measures among visitors, patients, and healthcare workers, improve transmission prevention within wards, and allocate appropriate isolation areas to prevent further spread among hospitalized patients.