The study took place from August 24, 2009, through December 16, 2009, largely before release of the pandemic infl uenza vaccine, and no participant was vaccinated during the study. Table 1 shows the number of patients who had confi rmed pandemic infl uenza infection (by PCR) and were treated in each of the hospitals. Characteristics of study participants are shown in Table 2 . The median participant age was 38 years (range 18-74 years); 27% were <30 years of age, 20% were 30-39 years of age, 25% were 40-49 years of age, and 20% were >50 years of age. Figure 2 shows the reverse cumulative distribution of fi rst serum antibody titers, according to age. We found no statistically signifi cant difference between the curves (p = 0.11 by ordinal logistic regression).