Single vaccinations with live lentogenic virus may produce a response in susceptible birds of about 4–6 log2,
but HI titres as high as 11 log2 or more may be obtained following a vaccination programme involving oilemulsion
vaccines. The actual titres obtained and their relationship to the type of protection and duration of
immunity for a given flock and programme are difficult to predict. Variation in HI titres may occur for
nonspecific factors, for instance due to the antigenic correlations, infection with other AMPVs (e.g. APMV-3)
may result in significant increased titres to NDV. The HI titre is also influenced by the characteristics of
antigen used. For instance, the use of the homologous La Sota antigen in the HI assay after vaccination with
this virus resulted in significantly higher titres than when heterologous Ulster virus was used (Maas et al.,
1998). Furthermore, reference antigens produced with historic strains may reduce the sensitivity of HI assay
when used for the detection of antibodies against ND viruses currently circulating. For this reason it is
important to investigate the antigenic relationships between the antigen used in the laboratory and current
circulating viruses, and between vaccine strains and reference HA antigens, to avoid misjudgements in
estimating serum antibody titres.