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 for 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 vaccination of SPF-chickens, resulted in significantly higher titres than the use 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 neutralization (VN) assays. The virus strain in the HI or VN test did not influence the resulting titres after Ulster vaccination, When HI antibody titres after 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 Sota antigen in the HI assay after vaccination with Clone30 or La Sota resulted in an overestimation of protective serum antibody titres. Also in commercially 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 as the European Pharmacopeia does not prescribe the antigen to be used in the HI test.