Abstract: Inactivated vaccines were prepared with three H9 subtype AIV isolated from Hebei, Henan and Shandong between 1998-2009. SPF chickens were vaccinated and 21 days later blood samples were collected for HI test. Five field isolates from Hebei, Henan, Shandong and Beijing were used to infect immunized chickens to examine the protectivity of the candidate vaccines made with above isolates. The HI test results showed that HI antibody levels against the H9 antigens triggered by the three candidate vaccines were apparently increased. Although there was certain degree of differences at the antibody titers raised with different vaccines, using homologous virus as antigen resulted in higher HI titers. The challenge experiments showed that H9 subtype AIV strains isolated at different time and places produced sound cross-protection. The study also demonstrated that the inactivated vaccine prepared with 1998 field isolate WD98 offered fairly good protection against current circulating H9 subtype AIVs.

Key words: H9 subtype avian influenza; circulating strain; vaccine; cross-protection