The use of homologous virus in the haemagglutination-inhibition assay after vaccination with Newcastle disease virus strain La Sota or Clone30 leads to an over estimation of protective serum antibody titres

We evaluated the influence of the use of the Newcastle disease virus (NDV)- strains Lister and La Sota in the haemagglutination inhibition (HI) assay f or the measurement of antibody titres after NDV vaccination. The use of the homologous La Sota antigen in the HT assay after Clone30 and La Sota vacci nation of SPF-chickens, resulted in significantly higher titres than the us e of the heterologous Lister virus, The mean difference was 1.4 on a log(2) scale (2.6-fold). A significant difference was also found in virus neutral ization (VN) assays. The virus strain in the HI or VN test did not influenc e the resulting titres after Ulster vaccination, When HI antibody titres af ter vaccination were related to VN titres measured with virulent Herts NDV or to survival after virulent challenge, it was found that the use of La So ta antigen in the HI assay after vaccination with Clone30 or La Sota result ed in an overestimation of protective serum antibody titres. Also in commer cially derived White Leghorns vaccinated with Clone30, significantly higher HI titres were obtained when La Sota antigen was used in the HI test. Our data have direct implications for potency testing of inactivated vaccines a s the European Pharmacopeia does not prescribe the antigen to be used in th e HI test.