13 LOHMANN TIERZUCHT › MANAGEMENT GUIDE 13 VACCINATION Vector vaccines Vector vaccines offer new opportunities for the development of control programs and eradication of diseases in poultry facilities. These are genetically engineered vaccines in which a gene from one organism, the donor, is inserted into the genome of another organ- ism, the vector, to elicit a protective immune response against both organisms. Vaccination with a vector vaccine results in an immune response against the vector but also against the antigen included expressed by the vector without using the actual disease agent. The vector persists in the vaccinated bird, express- ing continuously antigen of donor. The main viral vectors used for the develop- ment of vector vaccines are Herpes virus of turkey (HVT) and the Poxvirus among others. These viruses have genomes large enough to accept large inserts. The main advantage of using vector vaccines is that they eliminate the post vaccination re- actions typical of vaccines based on live or- ganisms. Vector vaccines eliminate also the possibility of interference with the maternal immunity means being effective regardless maternal antibodies status which plays a big role in in- fectious bursal diseases (IBD). The disadvantage of using vector vaccine instead of live vaccine is that the donor can- not spread within the vaccinated flock. Means birds missed during vaccination are not pro- tected. Using HVT-vectored vaccines at the hatchery provides protection against Marek diseases. However, it should be combined with Rispens Marek disease whenever is used in Layers or breeders. The HVT-vectored vaccines should never be mixed with another HVT virus. This warning includes regular HVT vaccine, HVT-Rispens combination products or other HVT vector vaccines.