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 Korean 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 hemagglutinin (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.