Seasonal influenza vaccine effectiveness in Thailand, 2022–2025: a multi-season test-negative design study

<title>Abstract</title> Background Seasonal influenza vaccination is a key public health strategy for reducing influenza-related morbidity and mortality. In Thailand, influenza vaccine effectiveness (VE) is routinely evaluated using surveillance data; however, VE may vary by season, virus subtype, and population subgroup. Continuous monitoring is essential to inform vaccination policy and program performance. Methods We conducted a retrospective observational study using a test-negative design to estimate influenza VE in Thailand from 2022 to 2025. Data were obtained from sentinel surveillance across nine hospitals in four regions, including Bangkok. Patients presenting with influenza-like illness (ILI) or severe acute respiratory infection (SARI) who were tested for influenza by RT-PCR were included. Cases were influenza-positive patients, and controls were influenza-negative patients. Vaccination status was based on self-report and verified through medical records or registry data, with vaccination defined as receipt of influenza vaccine at least 14 days prior to symptom onset. VE was estimated using mixed-effects logistic regression adjusted for age group, sex, comorbidity, obesity, patient severity, epidemiologic week, and influenza season, with hospital included as a random effect. Results A total of 26,623 participants were included, comprising 4,395 influenza-positive cases and 22,228 controls. Overall VE against laboratory-confirmed influenza was 46% (95% confidence interval [CI]: 37–54). VE was relatively consistent across seasons (42% to 52%). By subtype, VE was highest against influenza B (64%; 95% CI: 49–74), followed by A(H3N2) (46%; 95% CI: 34–56) and A(H1N1) (35%; 95% CI: 15–51). Subgroup analyses showed higher VE among adults aged 18–64 years (57%; 95% CI: 44–67) and lower, non-significant VE among young children and older adults. VE was higher among outpatients than among severe cases. Conclusions Seasonal influenza vaccination provided moderate protection against laboratory-confirmed influenza in Thailand, with consistent effectiveness across multiple seasons. These findings support continued influenza vaccination programs and highlight the importance of ongoing VE monitoring to inform public health policy and improve vaccination strategies, particularly among high-risk populations.