Investigation of an Influenza A(H3N2) Outbreak and Assessment of Vaccine Effectiveness at a Non-commissioned Officer Training Center, Saraburi Province, Thailand, 2025

Abstract

On 13 Oct 2025, an influenza-like illness (ILI) outbreak was reported at a non-commissioned officer training center in Saraburi Province, Thailand. We investigated to verify the outbreak, describe epidemiological characteristics, identify risk factors and the causative agent, estimate vaccine effectiveness (VE), and implement control measures. We conducted a retrospective cohort study among 903 students and staff. A suspected case was defined by fever (body temperature ≥37.5 °C) or a history of fever, plus cough and at least one other related symptom (sore throat, rhinorrhea, myalgia, headache, fatigue, or dyspnea), with onset between 1–28 Oct 2025. Data were collected via an online questionnaire, and specimens were tested using real-time PCR. Poisson regression with robust error variance estimated adjusted risk ratios (ARR). Of 887 respondents (98.2%), 159 suspected cases (attack rate 17.9%) were identified. Influenza A(H3N2) was confirmed. The epidemic curve, peaking on 11 Oct 2025, was consistent with person-to-person transmission, and the basic reproduction number was estimated at 0.81–1.10. Significant risk factors included close contact with a patient (ARR 1.22; 95% confidence interval (CI) 1.06–1.42) and sharing personal items (ARR 1.20; 95% CI 1.06–1.36). Handwashing before meals was protective (ARR 0.80; 95% CI 0.72–0.89). The VE against clinical illness was 19.1% (95% CI -20.0%–43.9%). This outbreak was associated with personal hygiene-related risk factors. The VE was low and not statistically significant, which may be consistent with known vaccine limitations against A(H3N2) strains. The outbreak rapidly subsided following the implementation of public health control measures.