Learning from the outliers: A longitudinal ecological study of social and spatial inequalities in older adult influenza vaccination and hospitalisation (Cheshire and Merseyside, UK, 2018–19 to 2023–24)

INTRODUCTION: Influenza and influenza-like illness (ILI) place a heavy burden on UK healthcare. Vaccination programmes are shown to be effective but impeded by socioeconomic and demographic inequalities in uptake and outcomes. This paper examines why some neighbourhoods show atypical patterns of vaccination and illness beyond just deprivation. METHODS: Electronic health records covering influenza seasons between 2018 and 19 to 2023-24 were used to explore trends in influenza vaccination and ILI among adults aged 50+ years. Multi-level negative binomial regression models were used to examine how vaccination uptake and neighbourhood-level deprivation were associated with ILI admissions, as well as to identify outlier neighbourhoods, which were further profiled. RESULTS: Most ILI hospital admissions occurred among adults aged 75+. Higher vaccination coverage was linked to fewer admissions, although this association weakened after adjusting for neighbourhood deprivation, which itself showed a strong social gradient with more deprived areas experiencing two to three times higher admission rates. Vaccination uptake was consistently lower in more deprived areas, with the steepest declines in uptake seen in younger age groups during 2023-24. The temporary provision of universal access of vaccinations for the 50-64 age group between 2020 and 21 and 2022-23 was associated with the largest increases in the least deprived areas. Additional neighbourhood factors predicted little of the residual variation in ILI beyond deprivation, though communal establishments, distance to nearest general practice and older housing showed some associations. CONCLUSIONS: Reducing influenza harms will require both boosting vaccination uptake in disadvantaged communities and addressing the wider structural determinants of health.