Biological characteristics of Marek's disease vaccine CVI-988 clone C1

Biological characteristics of Marek's disease virus (MDV) CVI-988 clone C, of importance for vaccine application, are described. CVI-988 clone C was shown to be nonpathogenic for highly MD-susceptible chickens and slightly more effective than prototype CVI-988 vaccine. During plaque purification and serial cell-culture passages, reductions were observed in the release of 'A' antigen from infected cell cultures, in spreading properties and in virus replication in vivo. Pre-licensing batches of CVI-988 clone C vaccine afforded excellent protection against challenge infection with virulent MDV and highly virulent MDV strains. Groups of chickens with bivalent (HVT/SB-1) vaccine-induced maternal antibodies were equally protected by a double dose of CVI-988 clone C vaccine. Field trials performed in the Netherlands and in the United States confirmed the safety and protective efficacy of monovalent CVI-988 clone C vaccine.