Sapporo city, which had a population of 1,880,863 in 2005 [28], is one of the ordinance-designated metropolis cities in Japan and is divided into ten wards as administrative units [32]. There is a substantial difference in the density of population among wards. There are also many commuters to school and work between wards during the daytime, which leads to the mass movement of people on a daily basis [28]. To simulate the situations of an influenza epidemic on a realistic basis, we adopted an IBM that can reflect the heterogeneous population structure for the targeted area, in which each individual was assigned his/her personal information, such as age, household, residence district, social activity group, and casual contact group. The age structure in Sapporo city [28] is shown in Fig. 1. The distribution of household size was determined on the basis of the National Census of Japan data [28], where each household was assumed to consist of at least one adult aged ≥19 years.
Fig. 1 Age distribution of Sapporo city [28]