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Title: The influence of different sexual-contact patterns between age classes on the predicted demographic impact of AIDS in developing countries.
Author: Anderson RM, Ng TW, Boily MC, May RM.
Journal: Ann N Y Acad Sci; 1989; 569():240-74. PubMed ID: 2698092.
Abstract:
A model is developed to describe the spread of HIV within heterosexual communities and the demographic impact of AIDS. The model combines epidemiologic and demographic processes and is designed to mirror the impact of AIDS in sub-Saharan Africa. Refinements on past work in this area include unequal probabilities for transmission from females to males and from males to females, the inclusion of an age- and sex-dependent sexual-partner choice function and distributed incubation plus infectious periods. Numerical studies suggest that unequal transmission probabilities (weighted to a greater probability from males to females than vice versa), and the tendency of males to choose sexual partners of the opposite sex younger than themselves, both act to increase the demographic impact of AIDS over that predicted with equal transmission between the sexes and partner choice restricted within given age classes. Analyses support the conclusions of past work that the epidemic will only have a small detrimental impact on the dependency ratio of a population (the ratio of dependents to working adults) even when a weighting is added to take account of the extra burden imposed by the care of adult AIDS patients. However, a small increase in the ratio can imply a significant rise in the number of dependents within the population. Stimulation studies of the impact of changes in behavior to reduce transmission highlight to the need to induce such changes as early as possible in the course of the epidemic in order to minimize its impact. Directions for future research are discussed emphasizing the need to acquire quantitative data on sexual habits and to construct models to represent heterogeneity in sexual behavior. A mathematical model of the likely demographic impact of acquired immunodeficiency syndrome (AIDS) suggest that, in the countries of sub- Saharan Africa, this impact can be expected to increase in the years ahead as a result of unequal transmission probabilities (weighted toward a greater probability from males to females) and the tendency of men to select female sexual partners from a younger class. The model refines earlier work in its consideration of predicted influence of human immunodeficiency virus (HIV) infection on the age structure and dependency ratio of the population, the significance of patterns of sexual contact between the age classes of the 2 sexes, and the impact of control measures such as health education and condom use. The assumptions of a doubling time of 1.5 years in the early stages of the epidemic and a 3 times greater probability of transmission from infected males to susceptible females rather than vice versa produce the observation that net population growth rate will change from positive to negative 25 years after introduction of the AIDS virus. The dependency ration is not significantly affected by AIDS due to the counterbalancing effects of deaths from vertical and horizontal transmission. On the other hand, if dependency ratio is defined to encompass the burden created by the care of AIDS patients, the disease-related ratio will increase over time. The most significant social factor in terms of the impact of AIDS on net fertility is the tendency of African men to choose female sexual partners at least 5 years younger than themselves. Finally, the numerical analyses indicate that the introduction of AIDS control measures 10 years after the onset of the epidemic will slow the progression of the epidemic and prevent a change in the population growth rate from positive to negative over a 100-year period.

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