The two-step cluster analysis procedure was used to explore the natural groupings within the respondents. First, the auto-clustering exploratory analysis was performed using the categorical variables of vaccination status, vaccination intent, vaccination history and the continuous variables of knowledge score and risk perception score. Of the 522 respondents, 64 were automatically excluded from the analysis due to missing values on one or more of the variables. Of the 458 respondents assigned to clusters, 195 (42 . 6%) were assigned to the first cluster, 143 (31 . Subsequently the analysis was performed using the combined categorical variables of vaccination status in the previous season (=yes) and vaccination history and the continuous variables of knowledge and risk perception scores. The results were auto-clustered into four groups but not explainable. The procedure was repeated with the cluster number fixed to 2 due to the values of BIC, ratio of BIC changes and ratio of distance measures. Of the total 188 vaccinated respondents, 12 were excluded due to missing values. Of the remaining 176 respondents, 107 (60 . 8%) were assigned to cluster 1 and 69 (39 . 2 %) to cluster 2. Vaccinated cluster 1 comprised those vaccinated only in the previous season, i.e. the newly vaccinated group and vaccinated cluster 2 contained those vaccinated in the previous season who had more than one previous vaccination, i.e. the continuously vaccinated group. Then, the same analysis was repeated for the unvaccinated respondents and two clusters emerged, i.e. unvaccinated cluster 1 (never vaccinated) and unvaccinated cluster 2 (used to be vaccinated).