a marek &# 39 ; s disease vaccine comprising an attenuated revertant virus derived as a clone of md11 / 75 / r2 or an antigenic component of the virus is characterized by a markedly reduced pathogenicity compared to the parent virus , without a comparable loss in immunogenicity . attenuated revertants of the invention are exemplified by md11 / 75 / r2 / 23 and md11 / 75 / r2 / 29 . these viruses or immunogenic components thereof can be formulated into monovalent and polyvalent vaccines .