The protection conferred against virulent Newcastle disease virus (vNDV) genotype VII by commercial double recombinant HVT vaccines and NDV live-attenuated vaccine as prime/boost vaccination regimens in commercial broiler chickens carrying maternally-derived antibodies (MDAs) against NDV.

Vaccines against vNDV are readily available and potentially protective; nevertheless, improved vaccination protocols are required to prevent clinical disease and discontinue the spread of the virus. This study assessed the effectiveness of two commercial recombinant herpesvirus of turkey vector vaccines (rHVT-NDV-IBDV) that express the fusion (F) protein of NDV and the virus protein 2 (VP2) of infectious bursal disease virus (IBDV). In commercial broilers with maternally-derived antibodies (MDAs) the efficacy of the rHVT-NDV-IBDV vaccines was evaluated when administered alone, in combination with live-attenuated NDV vaccine at one-day-old, or as part of a prime/boost strategy. The vaccinated birds were challenged with the genotype VIId vNDV strain (NDV/chicken/Egypt/1/2015) at various ages (14, 24 and 35 days). In comparison to sham-vaccinated control birds, the applied vaccination regimens were able to reduce or prevent mortality and virus shedding and clinical disease. Two weeks post-application, the two vector vaccines were serologically reactive with the MDAs and induced protective immune responses against the F protein. In the instance of early challenge at 14 days old, the combination of recombinant rHVT-NDV-IBDV with a live vaccine offered better protection and reduced virus shedding compared to the vector vaccine alone. Boosting with live NDV vaccine at 14 days old increased the protective effect of the vector vaccines and reduced virus shedding and the clinical index after challenge at 24 days old. Both combining and/or boosting with live vaccine together with the vector vaccine provided better protection and minimized virus shedding compared with vaccination with vector vaccine only in the instance of 5-week-old challenge.