H5-HK (A/HK/156/97), a clade 0 H5N1 isolate, and H5-VN (A/VN/1203/04), a clade 1 H5N1 isolate, represent H5N1 viruses isolated from the first human outbreak in Hong Kong in 1997 and a subsequent outbreak in Vietnam in 2004, respectively. Results shown in Fig. 5 indicate that H5-HK.tPA DNA vaccine-immunized rabbit sera showed high antibody responses recognizing both the autologous H5-HK HA antigen and the heterologous H5-VN HA antigen. However, the HA-specific IgG titers against the autologous H5-HK antigen were higher than those observed against the heterologous H5-VN antigen (p,0.05). Conversely, the H5-VN.tPA DNA vaccine elicited high level HA-specific IgG responses against autologous H5-VN and heterologous H5-HK HA antigens although the overall titers against its autologous H5-VN HA antigen may be higher (not statistically significant).