In the H5N1 vaccine group, pre-vaccine IgMs against recombinant H5-VN04 protein were detectable in the majority of study subjects. The IgM binding level increased significantly after vaccination (P 5 .006); however, only 8 (10.4%) of 77 subjects demonstrated a .2-fold increase (as shown by a .2-fold increase in optical density at 450 nm by ELISA) ( Figure 1A ). All of the study subjects had pre-immune IgG Abs that bound to H5-VN04 ( Figure 1B ). As expected, the binding to H5-VN04 (P , .001) ( Figure 1B ) and neutralization activity (MN titer) against H5N1 (the vaccine strain; P , .001) ( Figure  1E ) significantly increased after vaccination. IgGs against recombinant H3-A2/68 protein ( Figure 1C ) and H7-NL03 protein ( Figure 1D ) were also detected in the pre-immune serum samples. In contrast to H5-binding, the postvaccination IgG binding to H3 did not increase significantly (P 5 .11); however, it increased to reach statistical significance for H7 (P , .001), but the proportion of specimens with a .2-fold increase in H7 binding was significantly lower than that for H5 binding (1.3% vs 76.6%; P , .001).