SPECLAL ToPic 1: PROJECTING AIDS MoirTAulY _Ibx_ values of sexual behavior variables to match arbitrarily fixed levels of HIV prevalence in 1990: low prevalence (1 percent of the adult population), intermediate (3 percent) and high (6 percent). In these simulations, sexual behavior parameters are manipulated to achieve the specifled HIV prevalence, whereas other variables are set at one of three levels. These variables, selected because of their significance in the spread of the virus established in previous applications of the model (See, e.g. Bulatao, 1991) and because of known variations in conditions in Sub-Saharan countries, are: condom use, the prevalence of genital ulcers, and the intensity of blood screening. In addition, three different starting years (1970, 1975, and 1980) for the epidemic were used. The combination of every level of these variables, shown in table SI below, results in a total of 243 simulations, which were run from the start of the epidemic until 2025.

Other parameters of the model, such as fertility levels and trends, mortality from other causes, age structure, transmission rates, progression of children and adults from HIV to AIDS and death were selected on the basis of available Information from surveys and surveillance systems in Sub-Sahara African countries (U.S. Bureau of the Census, 1991). Especially useful in establishing parameter values were also "standard parameters" developed for HIV modeling exercises at the United Nations and at Family Health International (United Nations/World Health Organization, 1991; Family Health International, 1991). Table S2 shows a summary of the assumed parameter values used in the simulations.

Further assumptions were necessary regarding the course of the epidemic in the future. While intensive efforts to develop a vaccine are reported to be making some progress, at this time it does not appear likely that an effective vaccine will be available in the next few years. Arbitrarily, it is assumed that an effective medical intervention will become available in 2005, and that transmission of the virus resulting in new HIV infections will be cut in half each year thereafter.

It is also assumed that there is no interaction between HIV prevalence and fertility, or between AIDS mortality and mortality from other causes (other than that an individual who succumbs to AIDS cannot die of another cause). The evidence of individuals adjusting their fertility behavior based on the prevalence of HIV is very limited, and hypotheses for either an increase and a decrease in overall fertility have been proposed (World Bank, 1990; Ryder et al., 1991).

# _Sunulatin results_

the simulation output gives projections of HIV prevalence from 1990 to 2025 in the presence of varying levels of the cofactors, and of several resulting mortality indicators. The analysis of the simulation results consists of two steps. First, it is examined how well future HIV prevalence can be predicted from 1990 prevalence in these simulated scenarios. Second, the relationship between HIV prevalence and mortality indicators is examined. As AIDS mortality in Sub-Sahara Africa affects both adults and young children, both the effects of HIV level on future life expectancy at age 10 and on future mortality of children under five are analyzed.

Regression analysis is used to establish the relationship between 1990 prevalence and future prevalence, separately for males and females. In order to linearize trends over time in these data, logarithmic transformation of HIV prevalence are used. Table S3 shows the regression coefficients to predict future HIV prevalence from just three variables: 1990 HIV prevalence, 1990 HIV prevalence squared, and the year that the epidemic started. The model fits the data very well, especially early in the projection period, and the R<sup>2</sup> never drops below 0.85.