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Updated: Jan 11, 2026
An Optimized Hemagglutination Inhibition HI Assay to Quantify Influenza-specific Antibody Titers
Published on: December 1, 2017
Brand-specific influenza vaccine effectiveness in three Nordic countries during the 2024-2025 season: a target trial
Kristyna Faksova1,2, Emilia Myrup Thiesson1, Nicklas Pihlström3,4
1Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark.
Background:
Seasonal influenza causes substantial morbidity and mortality in older adults. While vaccination is recommended in Nordic countries for individuals aged ≥65 years, brand-specific effectiveness estimates are scarce but essential for regulatory decision-making. We evaluated brand-specific effectiveness of seasonal influenza vaccines against laboratory-confirmed influenza-related outcomes in Denmark, Finland, and one Swedish region during the 2024/2025 season.
Methods:
We conducted a nationwide cohort study using target trial emulation and linked health registries. Individuals aged ≥65 years were matched 1:1, comparing seasonal influenza vaccine recipients to non-recipients. Cumulative incidences of laboratory-confirmed influenza A and B, influenza-related hospitalisation, and death were assessed at 18 weeks post-immunisation. Vaccine effectiveness (VE) was calculated as 1 minus the risk ratio.
Findings:
A total of 1,164,686 matched pairs were included (mean age 75.4 years, standard deviation 7.3). Overall VE against influenza-related hospitalisation was 46.8% (95% CI 40.8-52.9), with risk differences of -21.3 (-28.9 to -13.8) and -99.3 (-119.6 to -79.1) in Finland and Denmark, respectively, per 100,000 vaccinated individuals. Brand-specific VE was 63.4% (38.1-88.7) for Efluelda Tetra (split virion-high dose), 48.2% (40.8-55.6) for Fluad Tetra (subunit standard-dose adjuvanted), 43.6% (23.7-63.6) for Vaxigrip Tetra (split virion standard-dose), and 30.6% (-7.8 to 69.1) for Influvac Tetra (subunit standard-dose). VE waned by -6.5 percentage points (-10.5 to -2.5) every 3 weeks.
Interpretation:
Seasonal influenza vaccines moderately reduced the risk of severe outcomes in older adults. Efluelda Tetra and Fluad Tetra appeared to offer favourable protection in their respective target groups, supporting their use in the 2025/2026 season. Annual monitoring using Nordic registries is crucial for informing evidence-based vaccination strategies and regulatory decisions.
Funding:
European Medicines Agency.
