Attack rates did not differ by index case-patient's sex (p = 0.225). However, attack rates differed signifi cantly by index case-patient's age (p = 0.022), and the lower attack rate was found for older index case-patients. There was no signifi cant difference in attack rates between close contacts of patients with imported cases and those with locally acquired cases (p = 0.282). No infection was found in close contacts exposed to index case-patients with subclinical infection, and the attack rate observed in close contacts exposed to symptomatic index case-patients during their symptomatic phase was higher (p<0.001). Almost identical attack rates were found among male and female close contacts (p = 0.808). However, attack rates were signifi cantly different among different age groups of close contacts (p<0.001), and the lowest attack rate was found for those >50 years of age. The attack rates were signifi cantly different across 8 contact types (p<0.001). The attack rate was 5.3% among spouses and 6.6% among other family members in the household, and was lower among other types of close contacts ( Table 3 ).The attack rate among passengers on the same fl ight was low, 0.9% overall, and 1 index case-patient transmitted infection to 0.19 close contacts on a fl ight on average (28 of 147), and the attack rate was higher among the passengers with longer fl ight times (>12 hours, p = 0.001). The attack rate among close contacts of service persons at public places was 0.2%, and 1 index case-patient transmitted infection to 0.01 close contacts of service persons on average (1 of 113). Nonpassenger close contacts with longer exposure duration (>12 hours), compared with those with shorter duration (>12 hours), recorded the higher attack rate (p<0.001) ( Table 3) .