Many HIV/AIDS (acquired immunodeficiency syndrome) models have been developed to help our understanding of the dynamics and interrelationships of the determinants of HIV (human immunodeficiency virus) spread and/or to develop reliable estimates of the eventual extent of such spread. These models range from very simple to very complex. WHO has developed a simple model for short-term projections of AIDS, details of which are presented here along with results obtained using the model to estimate and project AIDS cases for the USA, sub-Saharan Africa, and south/south-east Asia. WHO has also developed, based on the model described in this paper, a computer program (Epi Model), which will enable the user to easily change the values of any of the variables required by the WHO model.
PIP: To aid developing countries in short-term program planning, the World Health Organization (WHO) has devised a model capable of estimating likely trends and numbers of cases of acquired immunodeficiency syndrome (AIDS) over the succeeding 3-4 years. A human immunodeficiency virus (HIV) point prevalence estimate is used in combination with both the estimated year in which HIV transmission became widespread and the HIV infection curve during the epidemic period, and then these data are used to calculate annual cohorts of HIV-infected adults. The projected number of AIDS cases is obtained by multiplying each of these annual cohort estimates by the progression rates from infection to clinical AIDS. Basic to the WHO model is the assumption that cumulative HIV infections follow a sigmoid curve; it is further assumed that the distribution of the HIV infection over time will be skewed, with a long right tail. Application of this model to data from the US, sub-Saharan Africa, and South/Southeast Asia revealed some differences with prevailing estimates. According to the WHO model, the cumulative HIV incidence in the US by 1990 should be slightly less than the lower range (1 million cases) of the Centers for Disease Control estimate. For sub-Saharan Africa, the model yielded estimates of a cumulative total of 700,000 adult AIDS cases by the end of 1990 and over 2 million bases by 1994, with 1.75 million cumulative deaths--estimates that exceed official statistics by 10 times. Finally, the WHO model projects at least 60,000 AIDS cases in South/Southeast Asia by 1994, which, again, greatly exceeds official estimates. WHO has developed a computer program, Epi Model, based on this model that enables users to change any of the relevant variables.