Baseline
The baseline scenario was the situation with no intervention. The number of patients reached a peak at about 100 days after introduction of the initial patient; thereafter, the number decreased gradually, and the epidemic was eradicated after about 7 months (Fig. 2a). The effective reproductive number (R t) for 90 days after introduction of the initial patient, which was calculated following the formula of Germann et al. [12], was estimated to be 2.0 in the beginning and 1.6 about 10 days after the introduction (Fig. 2b). Infection spread throughout Sapporo city from Chuo ward for 1 month, decreasing just after the peak of the epidemic in the Kita and Higashi wards, which have a greater residential population than the other wards in Sapporo city, 4 months following its introduction [28] (Fig. 3). The infected cases and deaths in the entire population were assessed to be, on average, 58 and 0.2%, respectively, in 100-trial simulations.
Fig. 2 Baseline scenario. a Epidemic curves for the baseline scenario in 100-trial simulations (grey lines) and in the profile of the average (black line). b Changes in the effective reproductive number (R t) over a 90-day period
Fig. 3 State of a novel influenza infection spreading among wards in Sapporo city for the baseline scenario. a–f Graphic illustration of the sequence in which the epidemic develops and spreads from 30 to 180 days after the introduction of the initial patient. Patterns of brightness show the number of patients per wards (see legend)