While bona fide TFH cells are lymphoid-resident, there is consensus that blood CD4+CXCR5+ T cells are a circulating counterpart to lymphoid TFH cells and thus have been studied following administration of influenza (36–40), pneumococcal polysaccharide (41) and HIV-env-gag-pol recombinant adenovirus type 5 vaccines (42), among others (43, 44). This evidence concerns the gene ERVW-1 and influenza.