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Modelling HIV epidemics in the antiretroviral era: the UNAIDS Estimation and Projection package 2009
  1. Tim Brown1,
  2. Le Bao2,
  3. Adrian E Raftery2,
  4. Joshua A Salomon3,
  5. Rebecca F Baggaley4,
  6. John Stover5,
  7. Patrick Gerland6
  1. 1Population and Health Studies, East-West Center, Honolulu, Hawaii, USA
  2. 2Department of Statistics, University of Washington, Seattle, Washington, USA
  3. 3Harvard School of Public Health, Boston, Massachusetts, USA
  4. 4Department of Infectious Disease Epidemiology, Imperial College London, London, UK
  5. 5Futures Institute, Glastonbury, Connecticut, USA
  6. 6Population Division, Department of Economic and Social Affairs, New York, New York, USA
  1. Correspondence to Tim Brown, Population and Health Studies, East-West Center, 1601 East-West Road, Honolulu, HI 96848, USA; tim{at}


Objective The UNAIDS Estimation and Projection Package (EPP) is a tool for country-level estimation and short-term projection of HIV/AIDS 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 (ART).

Results By December 2008 over 4 million people globally were receiving ART, substantially improving their survival. EPP 2009 required modifications to correctly adjust for the effects of ART 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.

  • HIV modelling
  • national estimates
  • projection
  • surveillance
  • antiretroviral therapy

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited, the use is non commercial and is otherwise in compliance with the license. See: and

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  • Funding The authors acknowledge the support of UNAIDS for the ongoing development of EPP and Spectrum. National Institute of Child Health and Development grant no. R01 HD054511 supported the development of the uncertainty estimation methodologies in EPP.

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • We wish to make clear that the examples used here are illustrative in nature only. They may differ from actual national projections in the countries referenced, as they do not necessarily incorporate all data and information available at the country level. The views and opinions expressed in this paper are those of the authors and do not necessarily represent those of the United Nations. Its contents have not been formally edited or cleared by the United Nations.