Winter 2022–23 influenza vaccine effectiveness against influenza-related hospitalised aLRTD: A test-negative, case-control study

Influenza activity in the UK started early during the winter 2022-23 season, with most surveillance systems reporting high levels of hospitalisation, intensive care unit influenza admission and GP influenza-like illness (ILI) consultation rates. Laboratory confirmed positivity rates were comparable to those seen pre-pandemic, between the end of November 2022 and the end of January 2023, exceeding 25% as they did during the 2019-2020 season [1,2]. Annual vaccination against influenza is recommended in the UK to eligible higher-risk groups: adults ≥65 years(y); children and adults in at-risk groups (including during pregnancy); and, pre-school, primary and secondary school-aged children[3]. However, in 2022-23 the offer of seasonal influenza immunisation was extended to healthy 50-64y olds[4]. The vaccines used were quadrivalent, containing one influenza A(H1N1) virus, one influenza A(H3N2) virus, one influenza B/Victoria lineage virus, and one influenza B/Yamagata lineage virus[5]. Public health measures aiming to reduce the transmission of SARS-CoV-2 had affected the transmission of respiratory viruses like influenza during the previous two seasons, with the 22-23 season being the first one where social mixing returned to pre-pandemic levels. Systematic monitoring of the effectiveness of the seasonal flu vaccine (VE) is a public health priority as influenza activity returns to pre-pandemic levels.