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On 1 Aug 2017, the Bureau of Epidemiology was notified that five conscripts from a single battalion unit in Chiang Mai Province presented with influenza-like illness (ILI) in two days. A joint investigation was performed to confirm the outbreak, describe the epidemiological characteristics, and identify the source of infection and risk factors. Active case finding was conducted, and either nasopharyngeal or throat swab from 11 patients were collected. Environment and activities in the unit were studied, and a retrospective cohort study was conducted. An influenza outbreak occurred in the new conscript unit during 17 Jul to 20 Aug 2017, with 40.6% attack rate. Major symptoms were fever (100%), cough (83.8%) and runny nose (75.0%). Out of 86 clinically diagnosed cases, 11 were laboratory confirmed. None developed pneumonia. Influenza A(H3N2) was identified in all 11 specimens tested by reverse transcription polymerase chain reaction. Basic reproductive number (R0) among conscripts in the affected unit was 1.3 (95% CI = 1.24-1.38). Close contact with an ILI case was a significant risk factor for influenza infection (adjusted odds ratio = 3.56, 95% CI = 1.68-7.45). A strict protocol for daily screening and early isolation during the epidemic season could prevent influenza outbreaks in a military setting.
Keywords: influenza, outbreak, military, conscript
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