SPECIAL Topic 1: PROJECnNO AIDS MORTALuY Iv

# SPECIAL TOPIC I: PROJECTING AIDS MORTALITY

This section describes the methodology used to incorporate AIDS mortality in the projections and compares resulting demographic Indicators with those constructed if AIDS is not incorporated, as in previous editions of the World Bank's projections. The methodology makes use of an epidemiological model developed at the World Bank that simulates the spread of HIV, and the demographic model used to do the population projections (Bulatao and Bos, 1992). Simulations of the effect of current HIV prevalence on projected future mortality are used to derive equations that can be applied to populations with measurable levels of HIV infection with a minimum requirement for data.

## Epidemlological model

The epidemiological model, developed by Bulatao (1991) consists of a numerical simulation of three processes: the spread of HIV, the development of AIDS among those infected with HIV, and the progression form AIDS to death. The three processes are modeled separately for adults (those aged 15 years and over) and children. The model allows HIV to spread sexually between partners, through blood transfusions, infected needles, and perinatally between mother and child.

### _Seawl transmission_

In handling the sexual transmission of HIV, the model first divides all adults in the population into four sexual orientation groups: females (assumed to be all heterosexual), heterosexual males, homosexual males, and bisexual males. Females are assumed to choose partners from among heterosexual and bisexual males; heterosexual males from among females; homosexual males from among themselves and bisexual males; and bisexual males from females, homosexual males, and among themselves. Within each group, up to ten subgroups varying by sexual behavior can be defrned. Those who have developed AIDS are assumed not to be sexually active, and therefore are effectively in a no-sex subgroup.

Given some initial assumption about the proportion infected with HIV in a particular year, the number sexually infected in each subsequent year is calculated subgroup by subgroup. In its basic form, the calculation of the number of HIV seroconversions (0) In one year (t) in a given subgroup (s) of one sexual orientation group (r) is given by where M is the number of HIV seronegatives in the subgroup, P is the likelihood of adult survival from mortality unrelated to HIV, R is the proportion of potential sexual contacts, or agents, who are infective, T is the likelihood of transmission to group r through sexual contact, S is an average number for sex acts per partner, and F is the annual number of new partners. (Differeint values of F and S are the main factors distinguishing sexual orientation subgroups.) 0 and **OT** wil: be explained below.

The proportion of agents (R) infective to subgroup s is the weighted proportion across all other subgroups with which subgroup s has sexual contact who are asymptomatic but HIVseropositive (A). The weights used are the frequencies (F) with which these other subgroups take new sexual partners. The asymptomatic seropositives (A) in a given subgroup are calculated by summing up all those infected in previous years and subtracting those expected to progress to AIDS or to die from other causes.