Recombinant vaccines have been commercialized in recent years to provide more safe and efficacious vaccination than the existing ones. Negligible interference of MDA, absence of post-vaccination reactions and long-term duration of immunity are expected from vaccines using HVT as vector. We tested the onset and duration of immunity to NDV elicited by a rHVT-NDV vaccine administered at hatch as single ND vaccination or in combination with conventional live plus inactivated vaccine as part of a vaccination regime in commercial layers with MDA to MDV and NDV. The immunity to NDV was evaluated at regular intervals until 40 weeks of age by serology and challenges with a recent genotype VII velogenic NDV isolate. The effect of challenge on egg production at the peak of production and measurement of challenge virus excretion was also monitored.

Results to be presented indicated a reasonable early (74% and 95% clinical protection against challenge at 3 and 4 weeks of age, respectively) and long lasting, high level of immunity to ND, including protection against egg drop and significant reduction of challenge virus shedding.