The impact of an ageing population on pandemic risk in England

Background England's population is ageing rapidly, with those aged 80 years and older projected to increase 85% by 2047. Aging populations may change the dynamics of future pandemics, since older individuals face higher risks of severe outcomes and require a disproportionate share of early vaccine supply, for certain respiratory infections, despite having fewer social contacts. The net pandemic-risk implications of these competing effects cannot be resolved without dynamic transmission modelling. Methods We used an age-structured stochastic SARS-CoV-2 transmission model, previously calibrated to the real-time COVID-19 epidemic across seven NHS regions of England. Running the fitted model under the ONS principal population projection for 2047 as a counterfactual, we held all non-pharmaceutical interventions fixed and scaled vaccine supply to overall population growth. Nine counterfactual scenarios spanned three vaccine deployment strategies and three ONS migration assumptions. Findings The basic reproduction number remained largely unchanged by the projected age distribution. However, under the central counterfactual scenario, cumulative hospital admissions in the first two years of the pandemic were 674,004 - a 40.3% increase over the historic baseline - and hospital deaths increased by 49.4%, disproportionate to the 13.9% total population increase. Trajectories were similar before the Delta variant emerged but diverged thereafter, as a larger elderly cohort delayed vaccine protection in younger, higher-contact groups. Projected burden was robust to migration assumptions (664,451--689,833 admissions). Vaccine deployment strategy had substantially greater influence, with admissions ranging from 625,859 (optimistic age-scaled deployment) to 1,193,883 (non-scaled). Interpretations Population ageing will disproportionately amplify England's pandemic burden, driven primarily by the interaction between a growing elderly cohort and fixed age-ordered vaccine allocation, rather than by population growth alone. This vulnerability is largely independent of future migration uncertainty but is strongly modifiable through demographically-adaptive vaccine deployment planning developed before, not during, the next pandemic.