Antigenic characteristics of recent avian influenza A H5N1 viruses isolated from humans

Background: In February 2003, highly pathogenic avian influenza A H5N1 viruses reemerged in humans. Despite repeated outbreaks in domestic poultry in Hong Kong since 1999, this was the first isolation of H5N1 from humans since the outbreak in Hong Kong in 1997, which resulted in 18 human cases and 6 deaths. Methods: To better understand the antigenic relationship between the 2003 H5N1 human virus A/Hong Kong/213/03 (HK/213) and other H5 viruses, post-infection ferret sera or post-infection human sera were tested for reactivity by hemagglutination–inhibition and microneutralization assays with H5N1 viruses circulating in Hong Kong or elsewhere in Asia since 1997. Results: The H5N1 virus isolated from a 9-year-old male in Hong Kong was antigenically distinguishable from recent H5N1 viruses isolated from wild birds in Hong Kong and from the human 1997 H5N1 viruses, using post-infection ferret sera. Likewise, sera from this case patient, collected 22 days post-symptom onset, reacted to high titers with the homologous HK/213 virus, but gave eightfold lower titers with A/Hong Kong/156/97, and other H5 viruses. Conclusion: These results suggest that this recent human H5N1 virus is antigenically distinguishable from current and previously circulating H5N1 viruses from Asia, including the viruses previously isolated from humans.