It should be noted that, after subtraction of the background binding of the negative control protein (Trx), the binding affinity of the peptide P1 to the human antisera after vaccination increased only about one fold compared with the antisera before vaccination (Fig. 6, panel D) . This might be contributed to cross-reactive antibodies to other influenza subtypes pre-existing in the human serum subjects, given the conservative nature of the P1 sequence or the R1 region [13] .