When the extent of CD4 T-cell-mediated immunity before and after seasonal infl uenza vaccination was compared in the healthy donors enrolled in the study, a nonhomogeneous pattern of responses was detected (online Appendix Figure; available from www.cdc.gov/EID/content/14/1/121-appG.htm. After vaccination (t1), a 2-fold variation of the frequency of antigen-specifi c T cells higher than baseline was arbitrarily considered signifi cant. According to this threshold, an increased frequency of IFNγ-producing CD4 T cells specifi c for vaccine preparation was observed after vaccination in 5 (donors 8, 11, 17, 26, 42) of 21 donors (23.8%). A slight increase of frequency of the vaccine preparation-specifi c CD4 T cells was observed in 5 donors (donors 9, 12, 33, 36, 40; 23.8%); a mild-tosignifi cant decrease was observed in the remaining donors (n = 11; 52.3%).