Figure 4. Neutralizing antibody (NAb) responses in rabbit sera immunized with differently designed H7-HA DNA vaccines. Neutralization assays were measured against H7N1 pseudovirus expressing H7-HA from A/Netherlands/219/03 (H7N7) and H1 N1-NA from A/VietNam/1203/04 (H5N1), using rabbit sera collected at 2 wk after the fourth DNA immunization or relevant pre-bleed sera, as indicated. (A) Percentage neutralization titration curves. Each curve represents the mean percentage of neutralization with standard deviation of each group of 3 rabbits. The dashed line indicates the 50% neutralization cut-off. (B) Neutralizing antibody titers were measured at 50% inhibition (IC₅₀) of virus infection to target cells, shown as the geometric mean with standard deviation from each group of 3 rabbits. Statistical significance (P < 0.05) between vaccine groups is indicated