The screening against human plasma immunized by the 2009 A(H1N1) influenza vaccine was carried out essentially as described above. Among the 100 sequences obtained after FACS analyses, 74 peptides were found to be in-frame, and 51 were confirmed to be antigenic (RAYS ratio $2). It is interesting to see that the distribution of theses peptides is similar to that obtained using the mouse antisera, with only 4% of peptides in the HA1 region (87 and 37 aa in length), 4% across HA1 and HA2 (96 and 39 aa in length), and 92% in the HA2 region (100630 aa) (Fig. 2 , panel C). Using FCM, three representative clones, H-11 (residues 359-479 of HA), H-26 (residues 390-480) and H-45 (residues 423-480), all in the HA2 region, showed strong signals (RAYS ratio .5). Although only 2 antigenic peptides are in the HA1 region, a clone expressing H-1 (residues 108-194), which harbored the Sa antigenic site in RBD of the HA protein, yielded a RAYS ratio greater than 3 (Fig. 3, panel C) . Again, judged from the mean fluorescence intensities, the binding affinities of these displayed peptides were generally weaker than the whole HA1 (Fig. S2,  panel H) .