Although human immunodeficiency virus type 1 (HIV-1) and HIV-2 share modes of transmission, their epidemiologic characteristics differ and international spread of HIV-2 has been very limited. Recently, the prevalence of infection with HIV-1 but not HIV-2 has increased rapidly in different West African countries, where HIV-2 was probably present earlier. Among 19 701 women of reproductive age tested in Abidjan, Ivory Coast, between 1988 and 1992, the prevalence of HIV-1 infection increased from 5.0% to 9.2%, while that of HIV-2 declined from 2.6% to 1.5%. Differences in viral load may be responsible: reported results of virus culture and polymerase chain reaction assays suggest that at high CD4+ T-lymphocyte counts viral load is lower in HIV-2—infected than in HIV-1—infected persons; the efficacy of heterosexual and perinatal transmission of HIV-2 is less efficient than that of HIV-1 at this stage. At low (<0.20×109/L [<200/μL]) CD4+ T-lymphocyte counts, virus isolation is equally successful for both viruses, and the efficacy of heterosexual transmission is similar. Differences in HIV-1 and HIV-2 natural history are reflected in differences in viral load, that for HIV-2 being lower until immunodeficiency is severe. Differences in viral load throughout most of the natural history of infection appear to correlate with lower transmissibility of HIV-2 than HIV-1 and are the likeliest explanation for their markedly different global epidemiology.
De Cock KM, Adjorlolo G, Ekpini E, Sibailly T, Kouadio J, Maran M, Brattegaard K, Vetter KM, Doorly R, Gayle HD. Epidemiology and Transmission of HIV-2Why There Is No HIV-2 Pandemic. JAMA. 1993;270(17):2083-2086. doi:10.1001/jama.1993.03510170073033