Influenza hospital admissions prevented by vaccination: a transmission dynamic analysis of the 2022/2023 and 2023/2024 programmes in England

We analyse the impact of the seasonal influenza vaccination programme over the 2022/2023 and 2023/2024 winter seasons in England, the first two full seasons since the re-emergence of influenza following the COVID-19 pandemic. Our approach adapts an existing age and risk-stratified SEEIIR dynamic transmission model, by additionally fitting to hospital admission surveillance data and allowing for changes in mixing patterns during school holiday periods. After forward modelling what would happen in counterfactual scenarios where all or various parts of the vaccination programme were removed, we find that vaccination greatly reduces the influenza-related healthcare burden in England, with an estimated 38,600 to 46,500 hospital admissions averted in 2022/2023 and 55,100 to 64,700 in 2023/2024. Across both seasons, we find that the school part of the programme prevented the most admissions, while in terms of the per-dose impact, the preschool and school parts of the programme were the most effective. Our study highlights vaccine effectiveness (VE) against infection and onwards transmission as a key data gap. In particular, we explore the possibility of lower VE against infection in a sensitivity analysis, and find that the number of averted admissions is reduced to 12,500 to 19,100 in 2022/2023 and 21,900 to 34,700 in 2023/2024.