Early mathematical models varied in their predictions of the impact
of HIV/AIDS on population growth from minimal impact to reductions
in growth, in pessimistic scenarios, from positive to negative
values over a period of 25 years. Models predicting negative rates
of natural increase forecast little effect on the dependency ratio.
Twenty years later, HIV prevalence in small towns, estates, and
rural villages in eastern Zimbabwe, has peaked within the intermediate
range predicted by the early models, but the demographic
impact has been more acute than was predicted. Despite concurrent
declines in fertility, fueled in part by HIV infections (total
fertility is now 8% lower than expected without an epidemic), and
a doubling of the crude death rate because of HIV/AIDS, the rate
of natural population increase between 1998 and 2005 remained
positive in each socioeconomic stratum. In the worst-affected areas
(towns with HIV prevalence of 33%), HIV/AIDS reduced growth by
two-thirds from 2.9% to 1.0%. The dependency ratio fell from 1.21
at the onset of the HIV epidemic to 0.78, the impact of HIVassociated
adult mortality being outweighed by fertility decline.
With the benefit of hindsight, the more pessimistic early models
overestimated the demographic impact of HIV epidemics by overextrapolating
initial HIV growth rates or not allowing for heterogeneity
in key parameters such as transmissibility and sexual risk
behavior. Data collected since the late 1980s show that there was
a mismatch between the observed growth in the HIV epidemic and
assumptions made about viral transmission.