Combined Vaccination with Recombinant HVT-ND and Live Newcastle Disease Vaccines Accelerates Early Immunity, Limits Virus Shedding and Reduces Transmission

Background/Objectives: Effective control of Newcastle disease (ND) requires a reduction in virus shedding and transmission. While recombinant herpesvirus of turkey vectored Newcastle disease (rHVT-ND) vaccines induce strong systemic immunity, their ability to generate early mucosal immunity and limit transmission may not be optimal when used alone. Methods: Day-old layer chicks with maternally derived antibodies were vaccinated with the rHVT-ND vaccine alone or in combination with conventional live ND vaccines administered either at hatch or in a booster program. Birds were challenged with a genotype VII.2 velogenic ND virus strain at 3, 4 or 6 weeks of age. Humoral immune responses, clinical protection, challenge virus shedding and transmission to vaccinated contact groupmates were assessed. Results: All vaccination programs provided strong clinical protection (85–100%); however, combined vaccination strategies induced earlier immunity and reduced virus shedding more efficiently than the rHVT-ND vaccine alone. Booster vaccination further accelerated the onset of immunity and decreased shedding. By 6 weeks of age, differences among vaccination regimens had mainly disappeared, with all vaccinated groups showing minimal or no challenge virus shedding. Transmission to vaccinated contact birds was significantly reduced in the combined vaccinated groups, whereas unvaccinated controls efficiently spread the challenge virus and consequently caused high mortality in the contact groupmates. Conclusions: Both the rHVT-ND vaccine alone and in combination with live ND vaccines provided similar clinical protection against genotype VII.2 NDV challenge; however, combined vaccination programs accelerated immunity and more effectively reduced virus shedding and transmission, supporting their use for improved ND control, particularly in endemic regions.