Bulletin of Mathematical Biolooy Vol. 55,No.4, pp. 731-743, 1993. Printed inGreatBritain. 0092-8240/93 $6.00 +0.00 Pergamon PressLtd 9 1993 Society for Mathematical Biology COMPARISON OF TRANSMISSION RATES OF HIV-1 AND HIV-2 IN A COHORT OF PROSTITUTES IN SENEGAL CHRISTL DONNELLY, WENDY LEISENRING, PHYLLIS KANKI* and TAMARA AWERBUCH~" Department of Biostatistics and *Department of Cancer Biology, Harvard School of Public Health, Boston, MA 02115, U.S.A. SONJA SANDBERG Bunting Fellow at Harvard University and Mathematics Department, Framingham State College, Framingham, MA 01701, U.S.A. To explore the biological similarities and differences between the HIV-1 and HIV-2 viruses, we model the probability of male-to-female transmission of either HIV virus as a function of the number of sexual partners, the prevalence of the viruses and the infectivity per contact. Using maximum likelihood estimation theory and data from a prospective study of registered female prostitutes in Dakar, Senegal, we estimate and compare the infectivities of HIV-1 and HIV-2. Graphical goodness-of-fit methods are used to show that our model fits the data well. We find that in male-to-female transmission HIV-1 is significantly more infectious than HIV-2. This finding is consistent with other data from laboratory and epidemiologic studies comparing the biology of HIV-1 and HIV-2. 1. Introduction. In 1981 the first cases of the Acquired Immunodeficiency Syndrome (AIDS) were diagnosed in the United States (Centers for Disease Control, 1981). Soon after, a retrovirus (LAV or HTLV III, renamed HIV) was found to be the etiological agent of AIDS (Barr6-Sinoussi et al., 1983; Gallo et al., 1984). This virus was thought to be the only cause of the disease until a second retrovirus associated with an AIDS-like illness was discovered in western Africa in 1986 (Barin et al., 1985; Clavel et al., 1986; Clavel et al., 1987). HIV-2 was given its name to indicate its close relationship to HIV-1, the prototype AIDS virus. This was based on similarities in cell tropism, major antigenic cross-reactivity and genetic properties which include similar genome structure and approximately 50% nucleotide homology (Biberfeld et al., 1987; Kanki, 1987). Despite the similarities of HIV-2 to HIV-1 from a virological t Author to whom correspondence should be addressed. 731