Influenza pandemics occur less frequently, as a result of major changes in the surface glycoproteins of the virus (antigenic shift). The emerging novel influenza strain then easily spreads into an immunologically susceptible population. Consequently, pandemics are characterized by a shift in mortality toward the otherwise young and healthy 18to 35-year-old adults, with relative sparing of older patients, as evidenced by epidemiological analyses of the 1918 influenza A pandemic [6] . This is likely due to the persistence of immunological memory in older patients after previous exposures to H1-type viruses similar to the pandemic strain [7, 8] . The virulence of the pandemic strain may also play a role, as demonstrated by recent experiments with the highly fatal 1918 influenza strain [9] .