We first compared demographic differences between follow-up and lost-to-follow-up respondents with Pearson chi-square test while demographic differences of the respondents who completed both the baseline and follow-up survey and the general population [61] were assessed using Cohen's effect sizes [44] . Proportions were calculated to describe patterns of vaccination intention, reported vaccination uptake, and major reasons for taking or not taking pH1N1 vaccination. Structural equation modeling was then applied to examine the determinants of pH1N1 vaccination, vaccination intention and vaccination planning based on the extension of TBP. Mplus 6.0 for Windows (Muthén & Muthén, 1998 -2010 was employed because the model comprised dichotomous (vaccination status) and ordinal (vaccination intention) outcome variables. Before testing the full structural model, zeroorder correlations between the measures of related constructs were calculated. Confirmatory factor analysis was performed to assess the adequacy of the measurement model including perceived benefits of pH1N1 vaccination, concerns regarding adverse effects of pH1N1 vaccination, social norms regarding pH1N1 vaccination, anticipated regret and vaccination planning. To test the full structural model, all variables were entered into the model simultaneously. Mean and variance adjusted weighted least squares estimation was applied to evaluate the standardized parameters (beta, b). Since chi-square test is very sensitive to sample size and non-normally distributed data, several other model fit indices were evaluated including the Comparative Fit index (CFI), the Tucker Lewis index (TLI), and RMSEA. A CFI.0.90 and TLI.0.90 indicates a good fit. RMSEA less than 0.05 and one ranging between 0.05-0.08 respectively indicate a good and acceptable model fit [55] . Misfitting models were respecified guided by theoretical soundness and modification indices [55] . Missing proportions ranged from 0.1% for seasonal flu vaccination history to 5.5% for the item ''I have planned when and where to get my pH1N1 vaccination this winter''. There was no missing data for reported vaccination uptake. Missing data were handled with multiple imputation [62] .