We assumed that individuals would mix with each other randomly (in the household, during social activities, and in casual contact groups) and that individuals had a possibility of becoming infected when an infected person belonged to any of their groups. The average contact rate in a household was set to be higher than that for other groups, followed by that in social activity groups. The average contact rate in a playgroup was set to be higher than that in other social activity groups because children spent more time in close proximity to each other at a day nursery. The usual average contact rate in the low-risk casual contact group was set to be quite low because such contacts occur accidentally during outings [12]. On the other hand, the average contact rate in high-risk casual contact groups, such as while commuting on crowded trains or buses, was assumed to be 30-fold higher than that in the low-risk casual contact group. This was assessed by the average commuting time to school and work (an hour per day) [33]. The average contact rates for every group are tabulated in Table 2. Because it is difficult for symptomatic patients to follow their usual behavioral pattern [34], it was assumed that a symptomatic patient had a different behavioral pattern from the day after onset; a symptomatic patient retreats to the home, thereby exposing only his/her family [35].
Table 2 Daily average contact rates in the household, during social activities, and in casual contact groups
Group Infected Susceptible Contact rate (on average)
Household Child Child 0.6a
Child Adult 0.3a
Adult Child 0.3a
Adult Adult 0.4a
Social activity group
 Playgroup Child (3–6 years) Child (3–6 years) 0.25a
 Elementary school Child (7–12 years) Child (7–12 years) 0.0435a
 High school Child (13–18 years) Child (13–18 years) 0.0375a
 College Adult (19–22 years) Adult (19–22 years) 0.0315a
 Work Adult (19–64 years) Adult (19–64 years) 0.0575a
Casual contact group
 Low Anyone Child (0–4 years) 0.0000181a
Anyone Child (5–18 years) 0.0000544a
Anyone Adult (19–64 years) 0.000145a
Anyone Adult (65–years) 0.0002175a
 High Anyone Child (0–4 years) 0.000543
Anyone Child (5–18 years) 0.001632
Anyone Adult (19–64 years) 0.00435
Anyone Adult (65–years) 0.006525
aRefer to [12]