A LINKED RISK GROUP MODEL FOR INVESTIGATING THE SPREAD OF HIV

This paper describes a model that simulates the spread of HIV and prog ression to AIDS. The model is based on classical models of disease tra nsmission. It consists of six linked risk groups and tracks the number s of infectives, AIDS cases, AIDS related deaths, and other deaths of infected persons in each risk group. Parametric functions are used to represent risk-group-specific and time-dependent average contact rates . Contacts are needle sharing, sexual contacts, or blood product trans fers. An important feature of the model is that the contact rate param eters are estimated by minimizing differences between AIDS incidence a nd reported AIDS cases adjusted for undercounting biases. This feature results in an HIV epidemic curve that is analogous to one estimated b y backcalculation models but whose dynamics are determined by simulati ng disease transmission. The model exhibits characteristics of both th e disease transmission and the backcalculation approaches, i.e., the m odel: reconstructs the historical behavior patterns of the different r isk groups, includes separate effects of treatment and changes in aver age contact rates, accounts for other mortality risks for persons infe cted with HIV, calculates short-term projections of AIDS incidence, HI V incidence, and HIV prevalence, calculates cumulative HIV infections (the quantity calculated by backcalculation approaches) and HIV preval ence (the quantity measured by seroprevalence and sentinel surveys). T his latter feature permits the validation of the estimates generated b y two distinct approaches. We demonstrate the use of the model with an application to U.S. AIDS data through 1991.