Figure 1. Schematic representations of the study period and eligibility determination.

The bars indicate the number of persons with new infections per day in Sapporo. The stacked area graph shows the detection rate of mutant strains in Sapporo. The BA.2 period was defined as when the detection rate of omicron subvariant BA.2 exceeded 80%, and the BA.5 period was defined as when the detection rate of BA.5 exceeded 80% and until the end of notifiable disease surveillance in Sapporo.