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Methods and procedures for estimating HIV/AIDS and its impact: the UNAIDS/WHO estimates for the end of 2001

Walker, Neffa; Stanecki, Karen Ab; Brown, Timc; Stover, Johnd; Lazzari, Stefanoe; Garcia-Calleja, Jesus Mariaa; Schwartländer, Bernharde; Ghys, Peter Da

Epidemiology & Social

Background: The Joint United Nations Programme on HIV and AIDS (UNAIDS) and the World Health Organization (WHO) have produced country-specific estimates of HIV/AIDS biannually since 1997. These estimates are a primary source of information about the extent and spread of the HIV/AIDS epidemic and its impact. The importance of having comparable country-specific estimates of HIV/AIDS is growing as estimates are used to determine how international resources to fight HIV/AIDS will be allocated to countries.

Objectives: This paper describes the procedures and process used to make the 2001 round of UNAIDS/WHO estimates of HIV/AIDS. The paper focuses on the different approaches used to make estimates of prevalence in countries with generalized and low-level and concentrated epidemics as well as on new curve-fitting software that was developed to produce epidemic curves for each country. In addition, it presents the assumptions used (e.g. survival from infection to death, the rate of mother-to-child transmission) that are required to derive estimates of incidence and mortality in adults, as well as prevalence, incidence and mortality in children.

Conclusion: The paper describes the general process by which the estimation and modelling procedures have been refined and improved over time. The paper also discusses the limitations and weaknesses of the procedures and the data used to make the estimates, and suggests areas where further improvements need to be made.

From aJoint United Nations Programme on HIV and AIDS (UNAIDS), Geneva, Switzerland; bUnited States Census Bureau, Washington, DC, USA; cEast-West Center/Thai Red Cross Society Collaboration, Bangkok, Thailand; dThe Futures Group International, Glastonbury, CT, USA; and eWorld Health Organisation, Geneva, Switzerland.

Correspondence to: Peter Ghys, UNAIDS, 20 Avenue Appia, Geneva 1211, Switzerland. E-mail:

Received: 4 October 2002; revised: 6 February 2003; accepted: 19 March 2003.

© 2003 Lippincott Williams & Wilkins, Inc.