fitting observed HIV surveillance data on prevalence

The UNAIDS Estimation and Projection Package (EPP) is a tool for country-level estimation and short-term projection of HIV epidemics based on fitting observed HIV surveillance data on prevalence. This paper describes the adaptations made in EPP 2009, the latest version of this tool, as new issues have arisen in the global response, in particular the global expansion of antiretroviral therapy. Results By December 2008 over 4 million people globally were receiving antiretroviral therapy, substantially improving their survival. EPP 2009 required modifications to correctly adjust for the effects of antiretroviral therapy on incidence and the resulting increases in HIV prevalence in populations with high ART coverage. Because changing incidence is a better indicator of program impact, the 2009 series of UNAIDS tools also focuses on calculating incidence alongside prevalence. Other changes made in EPP 2009 include: an improved procedure, incremental mixture importance sampling, for efficiently generating more accurate uncertainty estimates; provisions to vary the urban/rural population ratios in generalised epidemics over time; introduction of a modified epidemic model that accommodates behaviour change in low incidence settings; and improved procedures for calibrating models. This paper describes these changes in detail, and discusses anticipated future changes in the next version of EPP.

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Editors’ notes: Getting ahead of the epidemic means knowing where it is and where it is going. For the UNAIDS Estimation and Projection Package (EPP) to model national epidemics effectively to inform policy and programming it must incorporate changes as the epidemic evolves and as our understanding of what influences its evolution increases. The modifications described here reflect factors such as the increasing numbers of people on antiretroviral therapy around the world – 5.2 million at last count. On treatment, people are living longer with HIV and that means that HIV prevalence will tend to increase. But antiretroviral therapy decreases viral load with the result that the infectiousness of individuals decreases and HIV incidence will tend to decrease. Among the changes in EPP therefore are not only the numbers of people on antiretroviral therapy in a country but also how many are on first-line versus second line regimens, the CD4 eligibility criterion that the country has adopted, the HIV progression rate in the absence of treatment (11 year survival versus a fast pattern of 9 years), and the distribution of antiretroviral therapy among defined sub-populations. Further, the 2009 EPP allows for changing urban/rural proportions over time. The shift to a focus on HIV incidence will be much appreciated by programme managers and policy makers who need to know the effects of current programming on the trajectory of their epidemic so that they can make timely changes.