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 until18 weeks and it is recommended that a booster vaccine be administered at 10 weeks.
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