Close Contacts among Student Contributing to an Influenza Outbreak in a Semi-boarding Sports School, Chiang Mai Province, Thailand, 2022

Abstract

On 11 Sep 2022, the Department of Disease Control was notified of a cluster of influenza-like illnesses (ILI) in a semi-boarding sports school in Sanpatong District, Chiang Mai Province, Thailand. The investigation objectives were to confirm the outbreak, describe its epidemiological characteristics, determine risk factors, and recommend control measures. A descriptive study followed by a retrospective cohort study was conducted. A case was defined as a person presenting at the school from 22 Aug to 20 Sep 2022 and having at least two symptoms compatible with ILI. We observed dormitory conditions and students’ activities and collected throat and nasopharyngeal swab specimens. We identified 113 cases, resulting in an attack rate of 26.5% among the screened population and 18.9% among the total population. Almost all cases were students and none developed serious illness. Of 11 specimens tested, nine were positive for influenza A (H3N2). Key risk factors included attending a sports program with adjusted odds ratio (AOR) 2.9, 95% confidence interval (CI) 1.2–6.8, and being in close contact with other cases (AOR 2.3, 95% CI 1.4–3.7). Dormitory overcrowding and close contact during football practice were observed. We recommended regular ILI screening, especially for sports students, and adjusting the dormitory environment to maintain physical distancing.