To investigate the protective efficacy of different antibody levels against Newcastle disease virus(NDV),chickens were vaccinated with inactivated recombinant genotype Ⅶ NDV-matched A-Ⅶ strain vaccine.The serological differences between two vaccine strains of A-Ⅶ strain and LaSota strain were compared by cross hemagglutination inhibition(HI)test,and protection experiments against challenges with standard virulent NDV strain F48E8 and virulent genotype Ⅶ strain JSC0804 were carried out in vaccinated chickens of different antibody levels.The results showed the mean HI antibody titer against A-Ⅶ strain of anti-A-Ⅶ strain hyperimmune sera was significantly higher(about 1.51og2)than that against LaSota strain(P＜0.01).However,the mean HI antibody titer against La Sota strain of anti-La Sota strain hyperimmune sera was slightly higher than that against A-Ⅶ strain but showed no significant difference(P＞0.05).This result suggested there were some serological differences between two vaccine strains.Under the experimental conditions,none of vaccinated chicken showed any clinical signs after being challenged intranasally and intraocularly.However,varying degrees of virus shedding were detected.The shedding rates generally decreased with the increase of antibody titers.When the serum HI antibody titers against A-Ⅶ strain of the vaccinated chickens reached ≥131og2(challenge with F48E8 strain)or ≥141og2(challenge with JSC0804 strain),virus shedding was completely inhibited.The virus shedding in the vaccinated chickens was detectable generally within 5 days post-challenge.This study elucidated the correlation between the serum antibody level and protection efficacy in vaccinated chickens,providing a basis for immune control of Newcastle disease.
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