Objective: To illustrate the magnitude of the impact of AIDS on projections of mortality, to explain the reasons for the differences in projections by major international organizations and to provide a simple approach to estimating the impact of AIDS on life expectancy.

Results: AIDS has already had significant impacts on mortality in a number of countries in the developing world and this impact is expected to grow substantially in the next 10 years. By 2005 the population of the most severely affected countries in Africa will be 13-59 million less than it would have been without AIDS. Life expectancy may decline by as much as 27% in these countries. Country specific projections made by the United Nations (UN) and the US Census Bureau differ significantly in their estimates of the impact of AIDS. The UN projects that AIDS will reduce the population growth of the most severely affected countries in Africa by 13 million by 2005 and 30 million by 2025, while the US Census Bureau projects the reduction to be four times larger (59 million by 2005 and 120 million by 2025). These differences are due largely to the use of different methodologies for projecting future levels of adult HIV prevalence. Other factors contributing to the different projections are different estimates of current levels of HIV prevalence and different assumptions about the length of the incubation period (from initial infection until death from AIDS) and the perinatal transmission rate. In addition to the number of deaths caused by AIDS, useful indicators of mortality include life expectancy at birth, the under five mortality rate and the life-time risk of dying from AIDS. An equation for estimating the impact of AIDS on life expectancy is presented and its use is illustrated.

Conclusion: It is clear that AIDS has already increased mortality significantly in many countries and will continue to do so in the coming decades. Uncertainty about current and future levels of HIV prevalence among adults leads to differences in the projections of future AIDS-related mortality. As data and projection methodologies improve, the differences in projections may be reduced for sub-Saharan Africa, but the growing epidemic in some of the largest countries of Asia may increase uncertainty about future global impacts.