Brand-specific influenza vaccine effectiveness in the elderly population in 2024/25 in Finland

Abstract Influenza vaccination coverage in the elderly in Finland has improved but still misses the EU's 75% target: 58% of those 65+ were vaccinated in 2024/25. They received the non-adjuvanted VaxigripTetra or, if aged 85+, the adjuvanted FluadTetra for free. To evaluate the influenza vaccination program, we aim to estimate brand-specific influenza vaccine effectiveness (VE) in the Finnish elderly in 2024/25. Using national register data, we followed a nationwide cohort of adults aged 65+ from 1 October to 21 March. The exposure and outcomes of interest were seasonal influenza vaccination, hospitalization due to influenza (ICD-10: J09-11) with laboratory-confirmed infection, and ICU admission during such a hospitalization. Using target trial emulation, we compared the vaccinated and unvaccinated in 1:1 age-, sex-, region-, and comorbidity-matched survival analyses to estimate overall and brand-specific VE after 6 and 18 weeks of follow-up starting from 2 weeks post-vaccination. The cohort totaled 1311727 subjects and 611174 matched pairs. By week 18, 191 unvaccinated and 96 vaccinated subjects of the pairs were hospitalized due to influenza. Overall, VE against hospitalization decreased from 62% (95% confidence interval: 41%-83%) at week 6 to 50% (37%-62%) at week 18, while VE against ICU admission was estimated at 63% (-19%-100%) and 80% (51%-100%), respectively. Against hospitalization, VE of VaxigripTetra was constant over time (58% [34%-83%] and 52% [39%-65%]) but VE of FluadTetra decreased from 84% (50%-100%) to 38% (2%-75%). Against ICU admission, VE of VaxigripTetra was 76% (38%-100%) at week 18. No FluadTetra recipient was admitted to an ICU. Both vaccines protected the elderly against severe influenza in 2024/25, a A/H1N1-dominated season. The adjuvanted vaccine, recommended particularly for those aged 85+, was initially more effective, but immunosenescence likely caused a decline in VE, suggesting the need for timely vaccination just before the epidemic onset. Key messages • In 2024/25, influenza vaccination in the elderly reduced the risk of severe influenza by 50% (95% confidence interval: 37%–62%). • The potential superiority of adjuvanted and other enhanced influenza vaccines must be further investigated carefully considering immunosenescence.