Inspired by the high specificity of P5 among the H1-subtype viruses, we developed an indirect ELISA assay using the P5 peptide to evaluate its performance as a diagnostic tool for H1 antibodies. The HI test was used as a reference method. As shown in Table 3 , the overall agreement of these two methods was 87%, showing that the two methods have good correlation (Pearson correlation coefficient R = 0.741). The sensitivity and specificity of peptide-ELISA versus HI test was 96.5% and 74.4%, respectively, indicating the potential of the peptide-ELISA method in detecting antibody against H1-subtype IAVs.