Data were analyzed by using SPSS version 11.5 (SPSS Inc., Chicago, IL, USA). Median and range values were calculated for continuous variables, and percentages were calculated for categorical variables. Differences in attack rates were compared between subgroups of close contacts by using the χ 2 test. For the signifi cant difference found in multiple subgroups, this test does not enable identifi cation of which multiple subgroups are signifi cantly different, only that across all the subgroups there are differences. The variables with p<0.10 in χ 2 test were included in multivariate analysis. Multivariate unconditional logistic regression analysis was conducted to determine risk factors associated with infection in close contacts. Backward logistic regression was conducted by removing variables with p>0.10. Odds ratios (ORs) and 95% confi dence intervals were calculated for potential risk factors of infection. The Hosmer-Lemeshow goodness-of-fi t test was used to assess the model fi t for logistic regression. All statistical tests were 2-sided, and signifi cance was defi ned as p<0.05.