seeking for STDs, and patterns of alcohol and illicit drug use (which influence sexual behaviors).

Of particular interest is the potential effect of a vaccine against HIV on the course of the epidemic. The efficacy of a vaccine can be expressed as the product of three factors (Blower and McLean, 1994): the vaccine take (the fraction of recipients in whom the vaccine induces any immunological effect), the degree of the vaccine (the reduction in susceptibility per sexual partnership among those in whom the vaccine takes), and the fraction of those ceasing sexual activity before the vaccine-induced protection wanes. Whether a vaccine could ultimately lead to eradication of HIV and if so, the extent of coverage necessary to achieve eradication depend on R0, the reproductive rate of HIV. Modeling results show that vaccines with moderate efficacy or those administered to a population with a severe HIV epidemic (as measured by R0) could not achieve eradication; for example, 100 percent of the population at risk would need to be vaccinated if R0 were 2.0 and the efficacy of the vaccine were 50 percent (Blower and McLean, 1994). These calculations assume no change in risk behavior.

Perinatal Transmission

The second major mode of HIV transmission in Africa is perinatal, which accounts for approximately 15 to 20 percent of all AIDS cases in sub-Saharan Africa, in contrast with 5 to 10 percent worldwide (Quinn et al., 1994). The large numbers of infected children in Africa are explained by the high proportion of women infected with the HIV virus and the large number of children each women bears. Serologic surveys of pregnant women in Africa find that between 6 and 30 percent are HIV-positive (U.S. Bureau of the Census, 1994c). Sub-Saharan Africa accounts for three of every four women who have been infected with HIV worldwide (World Health Organization, 1995b).

Perinatal transmission may occur in utero through transplacental infection, at the time of delivery, or through breastfeeding or other routes. The probability of mother-to-child transmission varies according to different studies: 27 percent in Kampala, Uganda; 30 percent in Kigali, Rwanda; 39 percent in Lusaka, Zambia; 39 percent in Nairobi, Kenya; 39 percent in Kinshasa, Zaire; and 42 percent in Brazzaville, Congo (Ryder et al., 1989; Hira et al., 1989; Lallemant et al., 1989; Miotti et al., 1990). In comparison, transmission rates have been lower in North America and Europe, ranging from 7 to 30 percent (Blanche et al., 1989; Rogers et al., 1989; Oxtoby, 1990; European Collaborative Study, 1991). Unfortunately, the results of various studies published to date are not strictly comparable because of differences in recruiting strategies for prospective studies and the criteria used to determine HIV infection among children. Nevertheless, risks of perinatal transmission reported in African studies appear to be generally higher than those reported in North American and European studies, probably because of large differences between the duration and intensity of breastfeeding by seropositive



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