To date, 3 infl uenza subtypes have produced pandemic disease in humans: H1N1 in 1918, H2N2 in 1957, and H3N2 in 1968 (4,11,12) . In 1997, during the avian infl uenza (H5N1) outbreak in Hong Kong Special Administrative Region, People's Republic of China, a cross-reactive cellular immune response induced by infl uenza (H9N2) was able to protect chickens from infl uenza (H5N1) (13) . Moreover, adults living in the United States who were never exposed to H5N1 subtype have shown cross-type cellular immunity to infl uenza A virus strains derived from swine and avian species (including the H5N1 subtype isolated in Hong Kong) (14) . Thus, speculation that cross-reactive T cells may decrease illness and death by reducing the replication of the new infl uenza virus, even if elicited by a different strain, is reasonable.