A Marek'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.
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