Abstract
Maternally derived antibodies (MDA) are a major problem for the application of NDV-vectored vaccines in commercial chickens with high levels of MDA, which could impair its efficacy. In this study, we assessed the effectiveness of different commercial vectored ND in broilers chicks with MDA against local heterologous NDV genotype VII. For this purpose, Two hundred and forty one day old chicks were divided into eight groups (30 chicks/group) (G1, G2, G3, G4, G5, G6 and G7) vaccinated with four different recombinant ND vaccines using different vaccination scheme. In addition, Group 8 (30 chicks) kept as a control non-vaccinated group. Efficacy of this type of vaccine is evaluated routinely by vaccination-challenge test. ELISA test were used for detection antibodies from collected sera of all chickens groups. Challenge test were applied at 31th day's old age of chickens. To calculate viral Shedding, cloacal swabs were collected from all groups and examined by real time RT-PCR. Obtained results for G8 (control) was recorded high antibody titer at one day but decreased gradually till become negative at 24 day old. While vaccinated groups exhibited antibody titers at 38 days old which were 3.6, 3.7, 4, 3.7, 3.66, 3.77, and 3.3 for G1, G2, G3, G4, G5, G6 and G7, respectively. Protection% of vectored ND vaccines groups ranged from 80 to 95 % despite inactivated ND vaccine recorded 60%. It was concluded that the recombinant ND vectored vaccines more potent than inactivated one in broiler chickens with MDA. Administration of rFowl-Pox+ND vaccine at 10 day exhibited higher immune response and conferred more protection and reduction of viral shedding in comparison to vaccinated at hatchery. Using rHVT+ND vaccine in one day old chicks confer optimum immune response and protection% in presence of MDA. Prime-boost vaccination strategy can overcome the problem of MDA in broiler chickens vaccination. So, it was recommended to measure the level of MDA in offspring to determine the ideal time for vaccination

Key words: Newcastle disease virus, Maternally-derived antibodies, Recombinant vector vaccine, Vaccine interference and challenge.