14th International AIDS Conference Barcelona, Spain — July 7-12, 2002

Int Conf AIDS. 2002 Jul 7-12;14:Abstract No. WeOrC1309

BACKGROUND: The country-specific estimates of HIV-1 prevalence and AIDS mortality produced by the Joint United Nations Programme on HIV/AIDS (UNAIDS) are widely used for both planning and advocacy purposes. To date, UNAIDS have used a simple curve fitting procedure, based on a gamma distribution of HIV incidence. However, this method lacks flexibility, is not designed to make predictions, and does not describe the mechanisms underlying the spread of infection. In interpreting the progress of the epidemic a transparent and flexible model is required to represent the course of the epidemic and to make short-term projections.

METHODS: A simple, flexible and easily comprehended model, which can be used globally and fitted to sentinel surveillance data was developed in a collaboration between UNAIDS and an advisory group. Prevalence is modelled using an intuitive epidemiological description including a start date, an initial rate of effective contacts, a proportion of the population at risk and a parameter representing the behavioural response. These 4 parameters are fitted while externally supplied values are used for background rates of fertility and mortality and progress from HIV infection to AIDS.

RESULTS: The model has been successfully applied for a large number of epidemics from various countries and risk groups within countries. Confidence in the validity of the epidemic curve depends upon the availability of seroprevalence data. Through maximum likelihood methods short term projections of the future course of the epidemic can also be generated.

CONCLUSIONS: Through the integration of a number of different modelling approaches a simple and universal model for the HIV epidemic has been generated which has been applied to the country specific estimates of HIV prevalence. Future developments of this model should include more emphasis on the impact of interventions and on methods of short term projection.

Presenting author: G P Garnett

1 Imperial College, London, United Kingdom.

2 UNAIDS, Geneva, Switzerland.

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