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Influenza vaccine effectiveness in Iceland 2014-2022: A test-negative design
Valtyr Thors1, Rafael Daniel Vias2, Kristin Björnsdóttir3
1University of Iceland, Faculty of Medicine, Iceland; Children's Hospital Iceland, Landspitali University Hospital, Iceland.
Vaccine
|March 20, 2025
Summary
Influenza vaccination effectiveness in Iceland varied by season and age group, with higher protection observed in children. Targeting vaccination in young children may effectively reduce disease transmission.
Area of Science:
- Epidemiology
- Immunology
- Public Health
Background:
- Annual influenza epidemics pose a significant health burden, especially for vulnerable populations like children and the elderly.
- Influenza vaccination is a key preventive measure, but its effectiveness can be inconsistent.
Purpose of the Study:
- To evaluate the influenza vaccine effectiveness (IVE) in Iceland over seven influenza seasons (2014-2022).
- To analyze IVE across different age groups, including children and individuals over 60 years old.
Main Methods:
- A test-negative design was employed using national registry data from 2014 to 2022.
- Vaccine effectiveness was calculated as (1 - Odds Ratio) × 100%, comparing vaccinated individuals to unvaccinated controls among those tested for influenza.
Main Results:
- Overall IVE was 28.1%.
- Vaccine effectiveness was significantly higher in children (61.1%) compared to adults (27.0%) and individuals aged 60+ (14.8%).
- Influenza A (H3N2) was the most prevalent strain, and IVE fluctuated seasonally, with the highest effectiveness in 2015-16 and lowest in 2016-17.
Conclusions:
- Influenza epidemiology in Iceland mirrors that of neighboring countries, with children being disproportionately affected.
- Seasonal variations in IVE are likely due to vaccine strain mismatch.
- Despite variable effectiveness, influenza vaccination averts numerous cases, and prioritizing vaccination in young children could enhance disease prevention and control.
