Protective HI and MN antibody responses induced by H5-HK.tPA and H5-VN.tPA DNA vaccines were further compared against either A/HK/483/97 or A/VN/1203/04 wild type viruses (Fig. 5 ). HI and MN titers were present in both HK.tPA and H5-VN.tPA DNA vaccine-immunized rabbit sera against Similar analysis was conducted with rabbit immune sera elicited by H5-VN and H5-AH HA DNA vaccines (Fig. 6 ). H5-AH (A/ Anhui/1/05), a clade 2.3 H5N1 isolate, represents the H5N1 virus isolated from a human outbreak in China in 2005 [26] . The cross reactivity between H5-VN and H5-AH immune sera and viruses was very similar to that observed above between H5-HK and H5-VN immune sera and viruses. For binding antibody responses, H5-VN rabbit immune sera had significantly higher recognition to its autologous H5-VN HA antigen than the heterologous H5-AH HA antigen (p,0.05). H5-AH rabbit immune sera elicited higher antibody responses recognizing the autologous H5-AH HA antigen than the heterologous H5-VN HA antigen (not statistically significant) (Fig. 6) . For functional antibodies, both HI and MN analyses showed preference for H5-VN and H5-AH rabbit immune sera against their respective autologous wild type viruses, A/ VN/1203/04 and A/Anhui/1/05 (p,0.05 or p,0.01) (Fig. 6 ).