Effectiveness of a trivalent inactivated split-virion influenza vaccine (Tri Fluvac) in Thailand: a phase IV test-negative case-control study

Background Thailand has invested in domestic influenza vaccine manufacturing to strengthen health security, yet real-world evidence on the effectiveness of locally produced vaccines is limited. This study aimed to evaluate the effectiveness of a domestically produced trivalent inactivated split-virion influenza vaccine (GPO Tri Fluvac) against medically attended outpatient laboratory-confirmed influenza infection following its introduction into the population. Methods We conducted a phase IV test-negative case–control study among adults (≥18 years) presenting with influenza-like illness (ILI) in Lampang Province, Thailand, during the 2024–2025 influenza season. Laboratory-confirmed influenza was identified by RT-PCR. Vaccination status was obtained from hospital information system records. Vaccine effectiveness (VE) was estimated using multivariable logistic regression. Influenza virus clades and subclades were identified through viral genome sequencing. The study was registered in the Thai Clinical Trials Registry (TCTR20240909002). Findings Among 4975 participants with ILI, 703 (14%) had laboratory-confirmed influenza. GPO Tri Fluvac vaccination was received by 103 (15%) of 703 influenza cases and 1611 (38%) of 4272 test-negative controls. The adjusted VE of GPO Tri Fluvac against medically attended outpatient laboratory-confirmed influenza was 73.1% (95% CI: 66.3–78.5). Subgroup VE estimates by age group and virus subtype were presented descriptively. VE against hospitalization could not be reliably estimated because of the small number of severe outcomes. Genome sequencing showed that most circulating viruses belonged to the same major clades as the corresponding Southern Hemisphere 2024 vaccine strains included in GPO Tri Fluvac. Interpretation GPO Tri Fluvac provided substantial real-world protection against laboratory-confirmed influenza among adults in Thailand. These findings support its use in the national influenza vaccination program and highlight the value of domestic vaccine production for regional health security. Funding Thailand Science Research and Innovation through the National Vaccine Institute and the Government Pharmaceutical Organization.