Risk of severe COVID-19 in unvaccinated patients during the period from wild-type to Omicron variant: real-world evidence from Japan.

Patients aged ≥80 had less reduction in risk of severe COVID-19 during the Omicron variant period, and a greater preventive effect of vaccines against severe COVID-19, compared to younger people. Our findings suggest that booster vaccination is effective and necessary for older people, especially aged ≥80.