Cell culture attenuation eliminates rMd5ΔMeq-induced bursal and thymic atrophy and renders the mutant virus as an effective and safe vaccine against Marek's disease

► Report the first safe gene deletion vaccine for Marek's disease in chickens. ► Elimination of lymphoid organs atrophy which make the vaccine safe. ► Is critical for poultry industry on future vaccine for Marek's disease. ► This paper serves as a model for other animal vaccines.