To evaluate the immune-protective effect of GX0101Δmeq bacterial artificial chromosome (BAC) which contains an infectious meq-null Marek’s disease virus genome, 1-day-old SPF chickens were reared separately in isolators with positive filtered air. On 1 day of age, chickens were immunized with 20 μg of GX0101Δmeq BAC suspended in PBS, and challenge infection with 500 PFU very virulent rMd5 were performed at day 5 and 12 post-immunization separately. During 90 days after challenge, all chickens were recorded and checked for necropsy. The protective index of the two vaccines used was 80 and 40 for CVI988/Rispens and GX0101Δmeq BAC, respectively, after challenged with the very virulent (vv) virus rMd5 at day 5 post-immunization. When challenged with rMd5 at day 12 post-immunization, the protection index of GX0101Δmeq BAC increased to 67%. Except that GX0101Δmeq BAC can confer protection against vv MDV, a delay in the development of MD could be observed in some chickens vaccinated with GX0101Δmeq BAC. On the other hand, compared with CVI988/Rispens, the rescue of GX0101Δmeq 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 MDV.

Key words: Infectious clone, Marek’s disease virus, Meq null, Protective immunity