Non-protective variants of Marek's disease vaccine viruses

Two variant Marek's disease-vaccine viruses failed to induce protective immune responses in chickens and replicated poorly in vivo. One of these, MDV/200D, was defective for growth at 41 degrees C, and clones from this stock had 38 degrees C/41 degrees C plating ratios as high as 10(4). The basis for the poor in vivo replicative ability was not determined.