The effect of maternal immunity, age at vaccination and doses or live vaccines on immune response to Newcastle disease.

The short life span of domestic fowls necessitates the use of early vaccination, but the response is affected by the immaturity of the immune mechanism and by the presence ofmaternal immunity: this results in an uneven immunity of the large and continually changing poultry population. This paper compares the response of groups of chicks from vaccinated and non-vaccinated hens after vaccination at various ages with Newcastle Disease vaccines of different pathogenicity (lentogenic and mesogenic strains). The results show that: 1. maternal immunity has a great effect on the pathogenicity of the vaccine viruses and the subsequent immune response; 2. the older the chicks are at the time of vaccination, the better is their immune response; 3. for vaccines of low pathogenicity the immune response can be enhanced by increasing the dose of the virus irrespective of the presence of maternal immunity.