An improved hybrid HIV/AIDS model geared to specific public health data and decision making

An improved version of a previously described compartmental dynamic model for the spread of the HIV virus and AIDS is presented in which the estimation of key parameters depends entirely on the use of good public health data. This means that practical applications to specific regions, using only local data, can be of great value to public health decision makers dealing with local problems. It is assumed that scientific support is available within an interdisciplinary operations research context. The improved model incorporates physicians' delays in reporting AIDS incidence, additional cases revealed by death certificate analysis, and a high-risk core group of HIV positives involving a limited residence time, followed by a low-risk group with extended residence, both groups leading to AIDS cases. The model is hybrid in character in the sense that the HIV infection process with large numbers is deterministic while the incubation process with small numbers initially is stochastic. Applications have been made to Switzerland. Key parameters estimated include, in particular, the actual sizes of the original core groups for gay men and intravenous drug users. Current numbers of circulating HIV positives are also obtained.