Repeated vaccination with the current MDV vaccines (CVI988 and Fc126) within 1 week can invoke two consecutive productive infections, which elicits superior protection against MDV than a single vaccination in term of longer temporary expansion of CD8+, CD4+, and CD3+ T cells, stronger proliferative activity of peripheral blood lymphocytes and higher levels of neutralizing antibody [104], suggesting that productive antigen supply after vaccination favors induction of superior immunity against MD. This evidence concerns the gene CD4 and infection.