We specified five categories of social activity groups that an individual may belong to according to age: playgroup (3–6 years), elementary school (7–12 years), high school (13–18 years), college group (19–22 years), and work group (19–64 years). The age-specific rates of the enrollment of college students and employment were determined based on data in the School Basic Survey [30] and the Employment Status Survey [31]. The size of social activity groups was assumed to be smaller than their actual size so that individuals in a group could come into close contact with each other and transmit the influenza virus among them.