To investigate further the presence of heterosubtypic BnAbs in serum, we probed serum samples for the presence of F10-like IgG Abs, as determined by inhibition of the binding of biotinylated F10 (Bio-F10) to H5-VN04 in a competition ELISA assay. We found that F10-like Abs that could compete for Bio-F10 binding were also present in prevaccination serum specimens, with 23 (29.9%) of 77 samples demonstrating .30% inhibition of Bio-F10 binding at 1:90 dilution ( Figure 1F) . Furthermore, the F10-like IgG titers increased significantly after vaccination (P , .001), and the majority of vaccinees (54 of 77) had a 1.5-fold increase. In addition, there is a weak but significant correlation between the F10-like IgGs and the MN titer against H5N1 in post-immune serum samples, as determined by Spearman rank correlation analysis (co-efficient 5 .44); however, these stem pocket-directed BnAbs do not reach high enough levels to render a strongly significant correlation.