Influenza vaccine effectiveness in Iceland 2014-2022: A test-negative design

Annual influenza epidemics cause heavy disease burden, particularly in children and frail individuals. One of the available preventive measures is vaccination although the vaccine effectiveness is inconsistent. The aim of this study was to evaluate the effectiveness of influenza vaccination (IVE) in Iceland during seven influenza seasons.

The study population consisted of all individuals tested for influenza during each season from week 40 to week 20 of the following year and data was collected from 2014 to 2022 using national registries. A test-negative design was used to estimate vaccine effectiveness, calculated as (1 - OR) × 100 %, where OR is the odds ratio of vaccination in test-positive cases relative to test-negative controls, including age group classification. Analysis was done for all studied influenza seasons apart from 2020 to 21 when no influenza cases were detected.

Confirmed influenza cases were 2929 of which 781 were in children and 1192 in patients older than 60 years. Influenza A, H3N2 was most common. The overall vaccine effectiveness (IVE) was 28.1 % (95 % CI: 20.2-35.3). For children, the VE was 61.1 % (95 % CI:40.6-74.5), for all adults 27.0 % (95 % CI: 18.5-34.6), and for people aged 60 years and older, 14.8 % (95 % CI 1.4-26.3). The IVE was highest in the 2015-16 season and lowest in 2016-17.

Influenza epidemics in Iceland are similar to other neighbouring countries and infections are common in children. VE varies between seasons, probably at least partly due to vaccine strain mismatch. Despite low VE in several seasons, it can be assumed that thousands of influenza cases were averted. Targeting high vaccine coverage in young children may be an effective way of preventing disease and transmission.