Controlled studies comparing the protective efficacies of memory CD8 T-cell subsets in parallel by AT into recipient hosts are not feasible in humans, because individual HCT patients differ in their genetics and progress of hematopoietic reconstitution, latent virus strain(s)/clinical isolates that can differ in host-cell tropism [1,67,68,118–121], latent viral genome load depending on the individual history of infection, time of virus reactivation onset and organ site of reactivation, as well as in the extent of inter- as well as intra-tissue virus spread. This evidence concerns the gene CD8A and infection.