Optimization of the protocols for double vaccination against Marek's disease by using commercially available vaccines: evaluation of protection, vaccine replication, and activation of T cells.

Revaccination against Marek's disease is a widespread practice in some countries. The rationale of this practice is unknown, and there is no consensus in the protocols. Recently, we have demonstrated that administration of the first vaccine at 18 days of embryonation followed by a more protective second vaccine at hatch (18ED/1d) reproduced systematically the benefits of revaccination under laboratory conditions. Here, we have used the same model to optimize the revaccination protocols by using currently available vaccines and to determine whether two features associated with Marek's disease vaccine-induced protection (activation of T cells and replication of vaccine virus) are involved in the revaccination protocols. Protection conferred by three revaccination protocols (turkey herpesvirus [HVT] 18ED/HVT+SB-1 1d, HVT 18ED/CVI988 1d, and HVT+SB-1 18ED/ CVI988 1d) was evaluated. Revaccination protocols also were compared with single vaccination protocols (HVT 18ED, HVT+SB-1 18ED, HVT+SB-1 1d, CVI988 18ED, and CVI988 1d). Our results demonstrated that it is possible to improve efficacy of the currently available vaccines by using them in revaccination programs. Administration of HVT 18ED/CVI988 1d and HVT+SB-1 18ED/CVI988 1d were the two protocols that conferred the highest protection against a very early challenge (2 days of age) with very virulent plus Marek's disease virus strain 648A. In a separate experiment, we evaluated vaccine replication and activation of T cells in single and revaccination protocols. Our results demonstrated that replication of the second vaccine, although decreased compared with single vaccination, could be detected at 3 days (HVT, CVI988) or at 6 days (SB-1). Administration of the first vaccine (HVT) at 18ED resulted in a high percentage of activated T cells. Administration of a second vaccine (either HVT-SB-1 or CVI988) at 1d resulted in increased intensity of MHC-II stain in activated T cells.