Cross-protective antibody responses induced by individual DNA vaccines expressing HA antigens from key H5N1 viral isolates Based on the above results, additional HA DNA vaccines with the HA.tPA insert design were produced by using codon optimized HA genes that encode the HA proteins from H5N1 viral strains A/Anhui/1/2005 and A/Indonesia/5/2005, both have caused human infection in recent years [26, 27] . Given the circumstance that H5N1 influenza antigen drifts have occurred since the first human outbreak in Hong Kong in 1997, it would be important to determine if H5 HA vaccines developed based on H5N1 viruses isolated at different epidemic time points can induce cross antibody responses against other H5N1 viruses.