By multivariate analysis, age and type of contact were the major predictors of infection ( (OR 3.42; p = 0.002) and 0-19 years of age (OR 7.76; p< 0.001) were at higher risk for infection. Other signifi cant independent risk factors associated with infection included being a household member of a person with an index case (OR 3.83; p<0.001), being exposed to index case-patients during their symptomatic phase (OR 1.86; p = 0.003), and exposure duration >12 hours (OR 1.83; p = 0.002). Similar risk factors were observed among aircraft passengers.