Risk factors for infection with 2009 H1N1 pandemic influenza for 667 participants of the study for whom paired sera were tested and for whom complete information was available.

Data for one individual missing vaccination status and for another two were missing education status.aAIC for individual univariate models relative to that of the best fit multivariate model A.bAIC for mutually adjusted multivariate models, relative to that of Model A.CI, confidence interval; HK, Hong Kong; KLN, Kowloon; NT, New Territories (outlying islands included in NT West); OR, odds ratio.