IFNG and viral infectious disease: Multiple factors could condition the T cell response to vaccination or to any new viral infection, such as the repertory of naive T cells, which declines substantially with age but also with the persistent activation of T cells by other chronic viruses, such as cytomegalovirus; deficient ability of cytotoxic T cells to induce an effector response mediated by IFN-γ and cytolytic granules production or the induction of antigen-specific regulatory T cells under subimmunogenic conditions (45), among others.