To determine if the vif tag could influence viral fitness and affect the outcome of head-to-head competitions in dual-infection cultures, we established competitions with two viruses with the same Env protein (NL4–3, CC1/85 cl.7 or CC101.19 cl.7) but a different vif allele (i.e., vifX or vifY). This evidence concerns the gene ERVW-1 and infection.