*Facts about the Marek’s Disease (MD) Vaccine* . *There are no disadvantages to giving your chickens this vaccine. The current vaccine is derived from a turkey herpesvirus that cross protects for mareks so the actual mareks virus isn't in the vaccine. Therefore, unvaccinated birds can't get sick from it. . *It will help prevent your birds from developing tumors if they become infected with the MD virus. . *Must be given before birds are exposed to the virus to be effective. Best given when chicks first hatch.