A simplified vaccination program elicits an immune response comparable to a complex standard vaccination program in commercial layers under field conditions

Newcastle disease (ND) is a notifiable avian disease responsible for several panzootics, which has resulted in the establishment of mandatory vaccination programs against the virus in several countries including Denmark. This study compared the immune response elicited in layers by the standard vaccination program for ND of a Danish commercial egg production facility with a simplified version of the vaccination program. A commercial flock of layers was followed for 77 weeks from hatching to culling. The flock was divided into two groups according to vaccination program and housed separately. One group received the standard vaccination program consisting of a vector vaccine, a live vaccine (administered twice) and an inactivated vaccine (standard vaccination program). The other group received a newly marketed vector vaccine and the inactivated vaccine of the standard vaccination program (simplified vaccination program). Blood samples were collected at regular intervals from 30 randomly selected layers in each group until culling (77 weeks of age) and analysed for ND antibodies by ELISA assays, which measured antibodies against the nucleoprotein or fusion protein, and hemagglutination inhibition tests. Both vaccination programs provided lasting antibodies until 77 weeks. The simplified vaccination program showed significantly higher fusion protein antibodies and a markedly earlier onset of immunity at five weeks of age (97-100 % seroprevalence) than the standard program. The standard vaccination program reached the same seroprevalence at 14 and 24 weeks of age based on fusion protein antibodies and HI titres, respectively. The inactivated vaccine elicited a boost in antibody titres in both groups, however, boosting with the live vaccine used in the standard vaccination program did not result in an increased antibody response. This might indicate that administering of a vector vaccine prior to a live vaccine inhibits the serological response to the live vaccine.