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Objective: To evaluate the immuno-protective effect of GX0101 deltameq-BAC containing an infectious meq-null Marek's disease virus genome., Method: One-day-old SPF birds were reared separately in isolators with positive filtered air. On day 1 of age, chickens immunized with 101 microg of GX0101A deltameq-BAC suspended in PBS, challenge infection with 500PFU very virulent rMD5 was performed at day 5 and 12 post-immunization separately. During 90 days after challenge, all bird were recorded and checked for necropsy. The samples of heart and liver were collected for histo-sections., Results: The protective index of the two vaccines used was 87 and 33 for CVI988/Rispens and GX0101 deltameq-BAC, respectively, after challenged with the very virulent virus rMd5 at day 5 post-immunization. When challenged with rMd5 at day 12 post-immunization, the protection index of GX0101 deltameq-BAC increased to 53%., Conclusion: Except that GX0101 deltameq-BAC can confer protection against very virulent Marek's disease virus, a delay in the development of Marek's disease could be observed in some chickens vaccinated with GX0101 deltameq-BAC. On the other hand, compared with CVI988/Rispens, the reconstruction of GX0101 deltameq-BAC in the body is a prerequisite for access to protection. Therefore, there is a blank period after immunization, which provides a chance for infection with the wild Marek's disease virus.