Influenza Vaccine Effectiveness Against Symptomatic Influenza in Primary Care: A Test Negative Case Control Study Over Two Influenza Seasons 2022/2023 and 2023/2024 in Ireland.

Abstract

Live attenuated influenza vaccine (LAIV) is recommended in Ireland for all children aged 2-17 years. Quadrivalent influenza vaccine (QIV) is recommended for all others eligible for vaccination, including those ≥ 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. We undertook a test-negative case control study within the Irish sentinel general practice surveillance network as part of the Vaccine Effectiveness Burden and Impact Studies (VEBIS) network. We compared influenza vaccination status among influenza PCR positive cases with influenza PCR negative controls, both with ARI presentations, of all ages. We estimated IVE using logistic regression adjusting for age, onset time, medical conditions and sex. In 2022/2023, there were 288 cases and 765 controls. In 2023/2024, there were 567 cases and 1832 controls. In 2022/2023, overall IVE was 42% (95% CI 9 to 64) and 50% (95% CI -30 to 83) in 2- to 17-year-olds. Overall IVE in 2023/2024 was 35% (95% CI 15 to 51) and 68% (95% CI 30 to 87) in 2- to 17-year-olds. Influenza vaccination reduced the risk of influenza among ARI patients presenting to general practice, demonstrating the benefits of vaccination, particularly among children. Promotion of the seasonal influenza vaccine to recommended groups, should remain a public health priority. Targeted vaccination campaigns for children promoting LAIV should emphasise the effectiveness of LAIV in children.