Improving projections at the country level: the UNAIDS Estimation and Projection Package 2005

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Abstract
Background: UNAIDS has developed the Estimation and Projection Package (EPP) as a tool for national programmes to use for making national estimates and short term projections of HIV prevalence. EPP provides direct input to Spectrum, which produces incidence, deaths, and AIDS impacts. Methods: The latest version, EPP 2005, includes substantial methodological improvements over the previous version. These include: (1) parallel, but unique, interfaces for generalised and concentrated epidemics; (2) use of maximum likelihood fitting procedures; (3) a new procedure, known as level fits, adjusting for expansion of national surveillance systems into lower prevalence sites; (4) provisions for handling turnover in at-risk populations, including the reassignment of HIV positive former members to lower risk populations; and (5) user-defined calibration to HIV prevalence levels from general population or other epidemiological surveys. Results: Following regional training in mid 2005, this new version has been applied by many national programmes to make their end of 2005 estimates of HIV infections. UNAIDS has combined these national estimates to form the 2005 global HIV and AIDS estimates. Conclusion: EPP 2005 is a substantial improvement over previous versions, forming a solid base for the next round of modifications. Proposed modifications for that next version are presented for the reader’s information.