Immune response to the simultaneous vaccination of day-old chickens with live and inactivated oil-based Newcastle disease vaccines.

THe immune response, as measured by the haemagglutination-inhibition test, to the simultaneous administration of live Hitchner B1 and 2 commercially available, inactivated, oil-based, emulsified, Newcastle disease vaccines at day-old is described. The response was monitored from day-old to 18 weeks, when the birds were challenged with a standardized virulent virus. It was found that the haemagglutination-inhibition titre fell below log2 5 when the chicks were 10 weeks of age. Challenge at 18 weeks yielded a mortality rate of 25% in the groups receiving both live Hitchner B1 and an oil-based vaccine in comparison with 94% in the group receiving Hitchner B1 alone. Simultaneous application of live and oil-based vaccines at day-old is conclusively insufficient to maintain adequate protection until 18 weeks and it is recommended that a booster vaccine be administered at 10 weeks.