In animals, exposure to 1 specifi c subtype of infl uenza A virus can also induce protective immunity against challenges with other subtypes. This heterosubtypic or crossprotective immunity could represent a key mechanism for facing, and limiting, new infl uenza outbreaks. In 1997, during the Hong Kong infl uenza (H5N1) outbreak, an immune response induced by an infl uenza virus (H9N2), being T cells but not antibodies, protected chickens from lethal infl uenza (H5N1) (13) . Moreover, adults living in an urban area of the United States have been described as having infl uenza-specifi c memory T cells that recognize epitopes of infl uenza A virus strains derived from swine and avian species, including the infl uenza (H5N1) strain involved in the Hong Kong outbreak in humans (14) .