Preliminary data from the 2009 H1N1 pandemic suggest a similar shift in age-related mortality. An analysis of 532 cases of 2009 pandemic H1N1 influenza A in the US, for example, has revealed that 60% of the cases occurred in patients not older than 18 years of age and that only 5% occurred in patients older than 50 years [10] . In the cohorts recently tested, the modest extent of immunological memory in older patients was confirmed by the presence of serum crossreactive antibodies to the pandemic H1N1 influenza A strain found in 33% of the adults older than 60 years of age versus 6% to 9% of the adults 18 to 64 years of age and none of the children [11] . Influenza attack rates during seasonal epidemics vary between 10% and 20% but can be much higher during pandemics. For example, an analysis of the pandemic 2009 H1N1 influenza A outbreak in La Gloria, Veracruz, found clinical attack rates of 29% in adults older than 15 years and 61% in children younger than 15 years of age [12] . However, these rates may be different in geographic areas of low population density.