H9N2 is the most common subtype of influenza viruses in Chinese chickens and thus causes great economic loss for the poultry industry, even under the long-term vaccination programs. Recent human infections with avian influenza virus revealed that H9N2 is the gene donor for H7N9 and H10N8 viruses that are infecting humans too. The crucial role of H9N2 viruses due to the wide host range, adaptation to both poultry and mammals and extensive gene reassortment. In China, which is regarded as an epicenter of avian influenza viruses, the H9N2 virus has been detected in multiple avian species, including chicken, duck, quail, pheasant, partridge, pigeon, silky chicken, chukar and egret

The H9N2 influenza virus can be transmitted by air droplet, dust, feed, or water. Chickens usually seemed to be healthy after the infection but some of them do show depression and ruffled feathers. The virus replicates itself in the trachea. It makes chickens more susceptible to secondary infections, especially Escherichia coli infections with a mortality rate of at least 10%. Also, the trachea or bronchi are easily embolized by mucus when the ventilation is poor, leading to severe respiratory disease and death.

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.