Characterization of a Chicken Embryo-Adapted H9N2 Subtype Avian Influenza Virus

The low pathogenic avian influenza (LPAI) caused by H9N2 virus has generated economic losses in the Ko- rean poultry industry since 1999, but field isolates could not be used for killed oil emulsion vaccine directly because of low productivity in embryonated chicken eggs (ECE). Therefore, we established a vaccine strain, KBNP-0028, by passage through ECE with allantoic fluid showing the highest volume and titer. KBNP-0028 possessed 158 N-glycan on hemag- glutinin (HA) as its parent strain, but acquired a 16 amino acid deletion in the stalk region of neuraminidase (16-del NA) as a result of balancing with the glycosylated HA. KBNP-0028 was sufficiently productive in ECE to be compared with the A/Puerto Rico/8/34 (PR8) strain and immunogenic enough to induce high antibody titers. The antibodies inhibited hemagglutination of recent field isolates as highly as they inhibited KBNP-0028. Genome analysis revealed that KBNP- 0028 carried no known mutation relevant to virulence in mammalian. In conclusion, KBNP-0028 is a promising vaccine candidate for prevention of the LPAI in the Republic of Korea.