First large-scale target trial emulation using linked Nordic registries estimated brand-specific influenza vaccine effectiveness in adults aged ≥65 years during the 2024-2025 season.
Efluelda Tetra (split virion-high dose) showed the highest vaccine effectiveness (~63%) against influenza-related hospitalization, followed by Fluad Tetra (subunit adjuvanted), highlighting the benefit of enhanced formulations in older adults.
Overall influenza vaccination reduced hospitalizations by nearly 47%, with a risk difference of up to -99 per 100,000 vaccinated in Denmark, supporting robust public health impact.
Vaccine effectiveness waned significantly over the 18-week follow-up, decreasing approximately 6.5 percentage points every three weeks, suggesting timing considerations in vaccine administration.
