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New model for AIDS in Third World.

New model for AIDS in Third World

African countries, with some of the highest birth rates in the world, may find their populations actually shrinking in coming decades, according to a new, computerized model of population dynamics. But scientists warn that the news is not good, as the decline will be due not to well-thought-out policies but to the AIDS epidemic, which is expected to overburden many of these countries' resources in the coming years.

While most experts agree that the AIDS-related social and economic toll in Africa will be great, not all agree that the new model is accurate. Critics say that despite higher death rates in developing countries, population growth will not be so severely affected. The debate highlights difficulties in desiging mathematical models to predict the course of the AIDS epidemic, and comes at a time when developing nations need accurate guidance as they consider different strategies for stemming the spread of the fatal disease.

The new model attempts to balance many factors affecting AIDS in developing countries, some of which have a much higher prevalence of the disease than does the United States. Designed by epidemiologists at London (England) University and Princeton (N.J.) University and described in the March 17 NATIRE, the model makes the alarming prediction that "AIDS is capable of changing population growth rates from positive to negative values over time scales of a few decades."

On a more positive note, it predicts little change in developing countries' "dependency ratios," defined as the proportion of a population aged less than 15 and more than 65 years old. The ratio measures the segment of society that depends upon others for support, and can serve as a predictor of socioeconomic stress on a society. Other researchers have stated that the dependency ratio might rise with the spread of AIDS. But the new model concludes that fatalities among non-elderly adults will be approximately equalled by the drop in the birth rate and by AIDS-related newborn deaths.

May and his colleagues inserted a range of "plausible values" for such uncertain parameters as mother-to-newborn AIDS transmission rates, average numbers of sexual partners and subsequent chances of infection, and the death rate of those infected. They found that for essentially all conditions the rate of spread of infection exceeded the rate of population growth, with varying amounts of time required for population growth to decline and drop below zero.

John Bongaarts, an AIDS demographer with the Population council in New York City, told SCIENCE NEWS that May's model "gets off track" because of its failure to statistically separate different population groups with different risks of infection. Bongaarts' own model, which he is designing for the Agency for International Development and which attempts to take such variables into account, predicts only a 1 or 2 percent decline in growth rates for developing countries. With current rates of 4 percent or higher, such a drop would still leave the countries with high growth rates.

"Of course," he adds, "even if you get only a 1 percent decline in the growth rate, that means a doubling of the death rate, so you have a lot more dead people and you get an enormous impact on the health care system." Computer models, he says, may help lessen that impact. "There are several ways that you can try to stop the epidemic. But before you start spending a lot of money, you can try it out on the computer."

"These mathematical models are essentially tools for thinking clearly," May agrees, cautioning that his model is still preliminary.
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Author:Weiss, Rick
Publication:Science News
Date:Mar 19, 1988
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