Efficacy of a combination vaccine containing MDV CVI 988 strain and EWT against challenge with very virulent MDV

With the emergence of very virulent Marek's disease virus (MDV) strains, va ccines based on herpesvirus of turkeys (HVT) appear to be not powerful enou gh to confer full protection, whereas in chicken flocks vaccinated with MDV CVI988 strain protective immunity sometimes is generated not early enough for full protection. For this reason combination vaccines containing INT as well as CVI 988 have been developed. In this paper the beneficial effect o f combining both types of virus strains in one vaccine for early protection is shown in a vaccination challenge experiment, in which one-day-old chick ens were vaccinated with suboptimal dosages of the monovalent vaccines and the same dosages in a combination vaccine. After 5 days the chickens were c hallenged with a very virulent MDV strain and subsequently observed for a p eriod of approx. 50 days. It appeared that the combination vaccine provided better early protection than the monovalent vaccines. In addition, the com bination vaccine was tested as vaccine administered in ovo. It appeared tha t after in ovo vaccination the vaccine conferred adequate protection agains t challenge with a very virulent MDV strain, 5 days after hatch, and that p rotection after in ovo vaccination was similar to that obtained after subcu taneous vaccination with the same combination vaccine.