H9N2 is a subtype of the species Influenza A virus (bird flu virus). H9N2 influenza viruses of domestic ducks have become established in the domestic poultry of Asia. Phylogenetic and antigenic analyses of the H9N2 viruses isolated from Hong Kong markets suggest three distinct sublineages. Among the chicken H9N2 viruses, six of the gene segments were apparently derived from an earlier chicken H9N2 virus isolated in China, whereas the PB1 and PB2 genes are closely related to those of the H5N1 viruses and a quail H9N2 virus A/quail/Hong Kong/G1/97 (Qa/HK/G1/97) suggesting that many of the 1997 chicken H9 isolates in the markets were reassortants.
The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), has issued a task order under an existing contract to Chiron Corporation of Emeryville, CA, for the production of an investigational vaccine based on an H9N2 strain of avian influenza virus that has infected humans and has the potential to trigger a modern-day pandemic. Over the years the H9N2 influenza strain caused illness in several children aged nine months to 5 years in Hong Kong with the latest occurring in December 2009.