Influence of serotype and virus strain on synergism between Marek's disease vaccine viruses

The enhanced protective effect (synergism) when certain Marek's disease (MD) vaccine viruses are combined has been widely used in the development of improved vaccines, but the mechanism is poorly understood. To better characterize the basis for synergism among MD vaccine viruses, three vaccine viruses from each of the three MD viral serotypes were evaluated alone and in various combinations for protection against early challenge with very virulent MD viruses in four replicate trials. Synergism seemed to be influenced by viral serotype because significant enhancement occurred frequently between viruses of serotypes 2 and 3 (five of nine bivalent vaccines positive), but rarely between viruses of serotypes 1 and 3 (one of nine bivalent vaccines positive) and 1 and 2 (one of nine bivalent vaccines positive), and was not detectable between viruses of the same serotype (none of nine bivalent vaccines positive). With some exceptions, the degree of synergism tended to vary inversely with the mean protective efficacy of the most protective component virus. Little effect of virus dose, virus dose ratio or type and route of viral challenge was noted. The combination of strains 281MI/1 (serotype 2) and WTHV-1/1 (serotype 3), both poorly protective as monovalent vaccines, consistently demonstrated high levels of synergism (over 300%) in antibody-positive chickens challenged 5 days post-vaccination with Md5 virus. This protocol may be a useful model system for further studies on mechanisms of synergism. However, mixtures that optimize synergism are not necessarily as protective as commercial vaccines.