Heterosubtypic immunity mediated by CTLs was described in naturally infected humans [88, 89, 102] . It is developed mainly against conserved epitopes of NP, M1 and NS1 [82, [103] [104] [105] [106] . Kreijtz et al. showed that virusspecific CTL developed in humans as a response to previous exposition to seasonal influenza A viruses of the H3N2 and H1N1 subtypes displayed considerable cross-reactivity also with avian influenza viruses (e.g. A/H5N1) [86] . Thus, it could be supposed that obtained pre-existing T-cell immunity in humans may help to decrease the severity of infection during a pandemic outbreak in comparison to those individuals, who lack cross-reactive influenza specific CTL populations [86, 88, 107] . Therefore, vaccines based on conserved CTL epitopes represent a reasonable approach to generate effective broadly protective cellular immunity against influenza viruses of various subtypes.