The Investigation of Influenza A (H3N2) Outbreak in a Social Welfare Center, Sisaket, Thailand: June - July 2025.

Abstract

An influenza A (H3N2) outbreak investigation was conducted among residents and staff at a social welfare center in Khukhan District, Sisaket Province, between June and July 2025. The objectives were to confirm the diagnosis and epidemiology, the distribution characteristics, and to identify potential causes and risk factors to guide effective prevention and control measures. Descriptive epidemiology and a retrospective cohort study were utilized. Data and active cases were actively sought and collected using case investigation forms. Nasopharyngeal swabs were obtained for laboratory confirmation, and environmental assessments were performed. Results: Out of 317 residents and staff, 205 met the case definition, resulting in an attack rate of 64.67%. These cases comprised 13 confirmed, 10 probable, and 182 suspected cases. The majority were male (136 cases) and female (36 cases), with a male-to-female ratio of 1.97:1. The median age was 49 years (range: 20–78 years). The five most common clinical manifestations were cough (60.49%), fever (55.12%), rhinorrhea (43.41%), headache (37.07%), and sore throat (28.78%). Six deaths were reported, representing a mortality rate of 1.81%. Multivariate analysis revealed that significant risk factors associated with the infection included sleeping in close proximity (OR_adj=1.92, 95% CI: 1.01-3.63), utilizing a communal bathtub on wet floors (OR_adj=17.55, 95% CI: 8.94-34.46), sharing drinking cups (OR_adj=3.42, 95% CI: 1.51-7.75), and dining in groups (OR_adj=3.12, 95% CI: 1.46-6.69). This outbreak was caused by Influenza A (H3N2) virus. The outbreak was successfully contained and terminated following the implementation of strict control measures, including patient isolation, surface cleaning and disinfection, strict enforcement of mask-wearing and hand hygiene, chlorination of water for shelter use, and vaccination.