Direct correlation between the titer of infectious bursal disease virus VP2-specific antibody and protection.

The titer of antibody specific to infectious bursal disease virus (IBDV) was measured by a new method and correlated with protection. Challenge was carried out in chickens with various antibody titers, which were measured using a latex agglutination-inhibition (LI) test, a rapid and easy technique for measuring IBDV VP2-specific antibody. When actively immunized chickens had LI titers of 1:2 or more, almost all were protected from subsequent challenge with highly virulent IBDV. In contrast, even when chickens passively immunized with maternally derived antibody had LI titers of 1:4, half were susceptible to the same challenge.