Direct and indirect impacts of COVID-19 vaccination in Slovenia in 2021: a retrospective modelling study

Abstract Country-specific estimates of vaccination impact are essential for public-health planning after the COVID-19 pandemic. Most national estimates rely on a static formula that captures direct protection only. We retrospectively quantified the direct and transmission-mediated indirect impacts of COVID-19 vaccination in Slovenia in 2021 using national surveillance data and an age-stratified extended SEIR model with vaccinated and unvaccinated compartments, age-specific mixing and observed vaccination uptake. The actual rollout reached 56% population coverage by the end of 2021 and was compared with counterfactual scenarios of no vaccination, lower and higher coverage, and vaccination restricted to people aged 65 years and older, under the assumption that nonpharmaceutical interventions and behaviour remained as observed. Compared with no vaccination, the model estimated that the actual rollout averted 35,936 hospital admissions (95% uncertainty interval [UI] 29,965–40,322), 8,137 intensive care unit admissions (7,263–9,027) and 5,248 deaths (4,170–6,356), corresponding to reductions of 71% (67–73), 73% (71–75) and 66% (61–70). Comparison with an older-adult-only strategy suggested transmission-mediated indirect protection for older adults, with 38% (19–45) fewer hospital admissions and 32% (23–38) fewer deaths under the actual rollout. These model-based, conditional estimates show what a calibrated transmission model adds to static estimates: healthcare burden, indirect protection and strategy comparisons.