In the analysis of all participants, we found that older age was associated with lower rates of pandemic (H1N1) 2009 infl uenza infection. We did not observe higher levels of preexisting antibodies against pandemic (H1N1) 2009 infl uenza with increasing age, which has previously been reported. However, results of other studies examining the relationship between seroprevalence and increasing age are confl icting (15) (16) (17) (18) . Immune mechanisms other than typespecifi c antibodies may be providing protection for older participants. Other possibilities are that older persons have older children who may be less likely to acquire or transmit infl uenza or that older participants were more conscientious with respiratory etiquette and hand hygiene; attempts to measure these factors were not included in this study.