Test-negative case-control study demonstrates substantial protection against influenza in adults, indicating strong real-world utility of domestically produced vaccines.
Key Points
To evaluate the real-world effectiveness of a domestically produced trivalent inactivated split-virion influenza vaccine (GPO Tri Fluvac) against medically attended outpatient laboratory-confirmed influenza infection in Thai adults.
Conducted a phase IV test-negative case–control study among 4,975 adults (≥18 years) presenting with influenza-like illness in Lampang Province, Thailand, during the 2024–2025 season (TCTR20240909002).
Influenza was confirmed by RT-PCR, vaccination status was ascertained from hospital information systems, and viral clades were identified using genome sequencing.
Vaccine effectiveness was estimated using multivariable logistic regression.
Influenza was confirmed in 703 of 4,975 participants (14%); vaccination was recorded in 103 (15%) of 703 cases versus 1,611 (38%) of 4,272 test-negative controls.
The adjusted vaccine effectiveness against medically attended outpatient laboratory-confirmed influenza was 73.1% (95% CI: 66.3–78.5).
Genome sequencing confirmed circulating viruses primarily belonged to the same major clades as the Southern Hemisphere 2024 vaccine strains, though vaccine effectiveness against hospitalization could not be reliably estimated due to low event counts.