I believe the Mareks vaccine given to chicks is actually live turkey Mareks. The turkey Mareks does not create symptoms in either turkeys or chickens. Vaccinated chicks can still acquire chicken Mareks and could still show symptoms, but they are typically milder and more recoverable. In most places every chicken has Mareks, whether vaccinated or not. When you consider how few chickens are crippled by the disease, even if not vaccinated, it seems like we should skip vaccination and cull any affected breeders, working toward making all chickens resistant.