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Attenuated Marek's disease herpesvirus: protection conferred on strains of chickens varying in genetic resistance.

Five strains of chickens differing in susceptibility to Marek's disease (MD) were vaccinated with the attenuated BC-1 isolate of MD herpesvirus and were subsequently challenged with virulent virus. Significant protection was conferred to all strains. However, when vaccination was at 1 or 2 days there were significant differences among strains in the incidence of gross lesions of MD. The ranking of strains for susceptibility to MD was not usually altered by vaccination. By comparison, differences in the susceptibility of strains was not observed among chickens vaccinated at 23 days and these were better protected than those vaccinated at 2 days. Vaccination at both 2 and 23 days showed no significant advantage over a single vaccination at 23 days.