Effector cells were characterized for their ability to release IFN-γ when cultured overnight in the presence of antigen. CD4 T cells were gated and analyzed for IFN-γ and IL-2 cytokine expression. A representative experiment with PBMC from a recently vaccinated healthy donor is shown in Figure 1 . Without stimuli, no cytokine production in CD4 T cells was detected (Figure 1, panel A) . However, the stimulation with the seasonal infl uenza vaccine preparation induced the production of IFN-γ by CD4 ef-fector T cells (Figure 1, panel B: 3 .2% of IFN-γ+ CD4+ T cells). Stimulation with inactivated infl uenza (H5N1) virus induced a CD4 T-cell response (Figure 1 , panel C: 1.0% of IFN-γ+ CD4+ T cells). Finally, some CD4 T cells specifi c for a pool of H5 and N1 (H5/N1) peptides were also generated in this donor (Figure 1 , panel D: 0.6% of IFN-γ+ CD4+ T cells). No IL-2 production was observed in these experimental conditions.