Investigation of an Influenza A (H3N2) Outbreak at a school in Phrai Bueng District, Sisaket Province (February 13 - March 3, 2025)

Abstract

An outbreak investigation of influenza was conducted in a school in Phrai Bueng District, Sisaket Province, during 13 February–3 March 2025. The objective were to confirm the diagnosis and the existence of the outbreak, characterize the magnitude of the problem and describe the distribution of cases by person, time, and place, identify the source of infection and modes of transmission, analyze risk factors among exposed individuals as well as protective factors and ultimately, to implement appropriate prevention and control measures. A descriptive epidemiological study was carried out by reviewing medical records and conducting active case finding through interviews with students and school staff. An analytical epidemiological study was performed to identify risk and protective factors among cases and controls. using a case–control study and Chi-square test, and Environmental assessment and laboratory investigations were also conducted. A total of 43 cases meeting the influenza case definition were identified, with an attack rate of 25.59%. The median age of cases was 11 years, and the male-to-female ratio was 1:1.38. Influenza A/B antigen rapid tests were positive for Influenza A in 3 cases, and real-time RT-PCR detected Influenza A/H3N2 viral RNA in 2 cases. A statistically significant protective factor was maintaining a distance of at least 1 meter between desks in classrooms (adj OR = 0.19, 95% CI: 0.06–0.58, p-value = 0.005). This study demonstrates that physical distancing, particularly in classroom settings, is an effective measure for reducing influenza transmission in schools. Following the implementation of control measures, the last case was reported on 25 February 2025, and no additional cases were identified during 7 days of continued surveillance.