Subject selection and inclusion criteria. A flow chart showing subject selection is provided in Figure 4 . A total of 12,965 subjects participated in the parent study [53] . All these subjects were Cantonese-speaking adults (aged$18) selected within households using a Kish Grid methodology, who were capable of and willing to answer a telephone interview. Additional details about inclusion criteria are available elsewhere [53] . Respondents in the 7 th , 9-12 th surveys of the 13 surveys comprising the parent study who, in the parent study agreed to be re-contacted and who had not received pH1N1 vaccine were invited to complete the baseline assessment for the present study. These five surveys (the 7 th , 9-12 th surveys) were selected because participants in these surveys had not had any follow-up contact either in the parent study or otherwise. This minimizes interview fatigue thereby improving response rates. These surveys were all of a comparable sample size, between 1,000-1,007 [53] . The five selected surveys were conducted between 21 July and October 23, 2009, and generated a representative [53] pool of 5,014 respondents of whom 61.4% (3,079/5,014) gave consent for further contact. From a list of the 3,079 subjects who agreed to be re-contacted, 1,648 calls were randomly selected and successfully made by a university telephone polling organization. Unanswered calls were tried at least four times at different hours and weekdays before being replaced by new numbers. Finally, a total of 1,511 (92%, 1,511/1,648) respondents agreed to participate in the baseline survey. Of these 78 (5%, 78/1,511) reported already having received pH1N1 vaccination and were therefore excluded as ineligible, leaving 1,433 respondents who completed baseline interviews.