Poultry vaccination directed evolution of H9N2 low pathogenicity avian influenza viruses in Korea.

Significant economic losses in the poultry industries have resulted from H9N2 low pathogenic avian influenza virus infections across North Africa, the Middle East and Asia. The present study investigated the evolutionary dynamics of H9N2 viruses circulating in Korea from 1996 to 2012. Our analysis of viral population dynamics revealed an increase in genetic diversity between the years 2003 and 2007, corresponding to the spread and diversification of H9N2 viruses into multiple genetic groups (named A and B), followed by a sudden decrease in 2007, which was associated with implementation of vaccination using a Clade A virus. Implementation of the H9N2 vaccination program in Korea has dramatically reduced the diversity of H9N2 virus, and only one sub-lineage of clade B has survived, expanded, and currently circulates in Korea. In addition, the antigenic drift of this new genetic group away from the current vaccine strain suggests the need to update the vaccine seed strain.