The economic burden of COVID-19 undervaccination: costs of hospitalisation, ICU admission, and death in Scotland.

The COVID-19 pandemic has imposed substantial financial and operational pressures on healthcare systems globally. While vaccines were known to reduce severe outcomes, their broader economic impacts, especially in publicly funded health systems, requires clearer quantification. This study evaluates the direct healthcare costs associated with COVID-19 outcomes in relation to vaccination status, using linked individual-level data from Scotland’s EAVE II cohort (June to September 2022). We defined undervaccination as the shortfall between recommended and received COVID-19 vaccine doses, stratified by age group. Retrospective cohort analysis employing Cox proportional hazards and linear regression models was used to assess the associations between undervaccination and the risk, frequency, and duration of hospitalisation, intensive care unit (ICU) admission, and death. We also conducted a counterfactual analysis to estimate averted costs and quality-adjusted life years gained (QALYs) under a full vaccination scenario.Findings demonstrate a strong dose–response relationship between undervaccination and severe COVID-19 outcomes. Among individuals aged 75+, those with higher undervaccination levels showed significantly increased hazard ratios for hospitalisation (up to 3.92 for sub-optimal level 2), ICU admission (up to 12.53 for sub-optimal level 2), and mortality (up to 6.63 for sub-optimal level 3). These elevated risks translated into substantial direct healthcare costs. Hospitalisation costs reached £4.7 million for the 75+ group and £2.6 million for the 16–74 group, while ICU costs totalled £70,489 and £246,486, respectively. Under a counterfactual full vaccination scenario, potentially avertable hospital costs were estimated at £1.4 million for the 75+ group and £0.5 million for the 16–74 group. Vaccination rates were lower among younger individuals, ethnic minorities, and residents in more deprived areas, highlighting persistent health inequalities. Of the total cohort of 4,992,498 individuals, 65.8% were fully vaccinated on 1 June 2022, while 34.2% were undervaccinated. These inequalities were not only clinically consequential but also economically costly: undervaccinated individuals generated more than five-fold the COVID-19 healthcare spend of their fully vaccinated peers. Universal full vaccination could have averted approximately £1.4 million (75+) and £0.5 million (16–74) in hospital costs, and a further £38,000 in ICU costs, during summer 2022.