CD8A and viral infectious disease: Vaccination of YFV-17D-immune subjects proved to be an insightful experiment because these individuals presumably have pre-existing antiviral CD4+ and CD8+ T cells to the yellow fever non-structural proteins encoded in the ChimeriVax vector, but would lack neutralizing antibody responses because the YFV-17D envelope and PrM genes in these recombinant viruses have been replaced by DENV2 or JEV envelope/PrM genes.