Influenza vaccine effectiveness in Ireland during two influenza seasons, 2022/23 and 2023/24

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Abstract Background Live Attenuated Influenza Vaccine (LAIV) is recommended in Ireland for all children aged 2-17 years. Quadrivalent influenza vaccine is recommended for all others eligible for vaccination, including those aged ≥18 years with underlying medical conditions and all aged ≥65 years. We aimed to estimate influenza vaccine effectiveness (IVE) against Acute Respiratory Infection (ARI) presentations to primary care due to influenza over two influenza seasons in Ireland, to inform vaccination recommendations and communication campaigns. Methods We undertook a test-negative design case control study within the national sentinel surveillance General Practice network, as part of the European Vaccine Effectiveness Burden and Impact Studies (VEBIS) network. We compared influenza vaccination status among influenza Polymerase Chain Reaction (PCR) positive cases with influenza PCR negative controls, amongst patients consulting with ARI. We verified vaccination status using the national influenza vaccination database. We estimated end of season IVE for 2022/23, and mid-season IVE (up to 7th April 2024) for 2023/24, using multivariable logistic regression and adjusting for age, onset time, medical condition, and sex. Results In 2022/23, there were 288 cases and 765 controls. In 2023/24 there were 515 cases and 1,482 controls. Overall vaccination coverage was 23% in both seasons. Coverage in 2-17 year-olds was 10% in 2022/23 and 12% in 2023/24. In 2022/23, overall IVE was 42% (95%CI 9-54) and 50% (95%CI -30-83) in 2-17 year-olds. Overall IVE in 2023/24 was 37% (95%CI 16-54) and was 67% (95%CI 28-87) in 2-17 year-olds. Conclusions Influenza vaccination reduced medical attendance to primary care with ARI due to influenza, demonstrating benefits of vaccination, particularly among children. Influenza vaccination should continue to be offered and promoted to recommended groups, including children. Key messages • This study suggests that influenza vaccination protected population health and the health service from medical attendance to primary care with acute respiratory infection due to influenza. • Higher influenza vaccine effectiveness was observed in children aged 2-17 years. These findings can inform influenza vaccination programmes with LAIV for children in Ireland and internationally.