Human sera from the same donors were tested for HI activity against both vaccine and infl uenza (H5N1) preparations and for neutralization activity against infl uenza (H5N1) virus. Individual titers are reported in Figure 3 . A 4-fold rise in HA antibody titer is considered noteworthy, and after vaccination most donors (28/38; 73.7%) showed a noteworthy rise of HI titers against vaccine preparation, as indicated by an asterisk (Figure 3, top panel, black bars) . HI titers against infl uenza virus (H5N1) remained at undetectable levels after seasonal vaccination (data not shown), but a rise of neutralization titer >20-fold over baseline was observed in 13 (34.2%) of 38 donors ( Figure 3 , bottom panel, asterisk). All but 1 study participant also responded to seasonal vaccination by a rise in HI titers against vaccine preparation. One donor (21) showed high titers against the H5N1 subtype in NT but a low HI titer against vaccine, a unique situation in the study population. However, antibodies to both anti-infl uenza (H5N1) and infl uenza vaccine are raised by vaccination. Our fi ndings indicate that seasonal vaccination can raise neutralizing immunity against infl uenza (H5N1), which shows the existence of an antibodydependent cross-type immunity. No correlation between infl uenza-specifi c CD4 T cells and humoral responses was observed, which suggests that this type of antibody response was mainly CD4 T-cell independent.