Antibody response to strain combinations of Newcastle disease virus as measured by hemagglutination-inhibition.

Differences in antibody response to three Newcastle disease virus (NDV) strains--B-1, LaSota, and Ulster--were investigated using the hemagglutination-inhibition (HI) micro-titer test in chickens hatched from ND-immune and unimmune flocks. When used singly as primary vaccines, the Ulster strain stimulated the lowest antibody response of the three in both immune and unimmune (susceptible) chickens. Subgroups of each of the primary-vaccinated groups were revaccinated with each of the three strains. Ulster-vaccinated chicks, revaccinated with Ulster, gave the poorest booster response. All other revaccination combinations gave a significant titer increase, though some were better than others. It is suggested that the Ulster strain as primary vaccine followed by booster does of B-1 or LaSota will induce a higher antibody response (i.e., immunity) in susceptible chicken populations with less risk of a post-vaccination reaction.