Overall, we found that 17.7% of participants had serologic evidence of pandemic (H1N1) 2009 virus infection after the peak of the outbreak. This proportion refl ects the observed 16% seroprevalence in adults in Melbourne (14) . These rates are lower, however, than the 31.7% antibody positivity found in South Australia during a prelicensure study of pandemic infl uenza vaccine in July 2009, which excluded subjects with confi rmed or suspected pandemic (H1N1) 2009 infl uenza (15) . This difference in titers may have refl ected geographic differences in infection rates or differences between the populations sampled.