A stochastic individual-based model of SARS-CoV-2 transmission in a simulated US population of 5 million, stratified by age and risk group and calibrated to hospitalisation incidence from May 2024 to April 2025, compared vaccination timing, eligibility and dosing frequency. Annual universal vaccination in July gave 441 hospitalisations per 100,000 (95% UI 423-463) among adults aged 65 and over, against 578 (553-601) with no additional vaccination; if timing were unchanged, a hybrid universal-annual plus second dose for those 65 and over and immunocompromised adults gave 460 (440-487). The modelled summer advantage disappeared when coverage fell by 5-10 absolute percentage points, and benefit varied by regional seasonality.
Why it is interesting: Quantifies how much of COVID-19's residual hospitalisation burden tracks vaccination timing rather than eligibility, now that US transmission peaks in summer across much of the South and West.