ERVW-1 and infection: The boosting with two recombinant OPVs, one expressing SIV Gag and one expressing HIV Env, was based on the rationale of expanding responses primed by the DNA and, in the case of the Env immunogen, covering the V1V2 domain to stimulate antibody responses against this region, whose binding antibodies have been shown to correlate with protection from infection (5, 33).