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Background
In 2007, advances in the methodology of estimations of HIV epidemics applied to an expanded range of country data have resulted in substantial changes in estimates of numbers of persons living with HIV worldwide. However the qualitative interpretation of the severity and implications of the pandemic has altered little. The estimated number of persons living with HIV worldwide in 2007 was 33.2 million [30.6-36.1 million], a reduction of 16% compared with the estimate published in 2006 (39.5 million [34.7-47.1 million]). (UNAIDS/WHO, 2006) The single biggest reason for this reduction was the intensive exercise to assess India’s HIV epidemic, which resulted in a major revision of that country’s estimates. Important revisions of estimates elsewhere, particularly in sub-Saharan Africa, also contributed. Of the total difference in the estimates published in 2006 and 2007, 70% are due to changes in six countries: Angola, India, Kenya, Mozambique, Nigeria, and Zimbabwe. In both Kenya and Zimbabwe, there is increasing evidence that a proportion of the declines is due to a reduction of the number of new infections which is in part due to a reduction in risky behaviours.
Because estimates of new HIV infections and HIV-associated deaths are derived through mathematical models applied to HIV prevalence estimates, new estimates of HIV incidence and mortality in 2007 also differ substantially from earlier assessments. It is emphasized that these differences between estimates published in 2006
and those published in 2007 result largely from refinements in methodology, rather than trends in the pandemic itself. For this reason, it is inappropriate to draw conclusions by comparing 2007 estimates with those published in 2006. However, the methodological revisions have been applied retrospectively to all earlier HIV prevalence data, so that the estimates of incidence, prevalence and mortality from earlier years in the current report allow an assessment of trends over time.
The AIDS epidemic update reports on the latest developments in the global AIDS epidemic and has been published annually since 1998. The 2007 edition provides the most recent estimates of the epidemic’s scope and human toll and explores new trends in the epidemic’s evolution. This is a joint UNAIDS and WHO report and the estimates produced by the UNAIDS/WHO Working Group on Global HIV/AIDS and STI Surveillance are based on methods and parameters that are informed by the UNAIDS
Reference Group on HIV/AIDS Estimates, Modelling and Projections. These estimates are also based on work by country analysts in a series of 11 regional HIV estimates workshops conducted in 2007 by UNAIDS and WHO. The process and methodology used by UNAIDS and WHO were reviewed and endorsed by an International Consultation on AIDS
Epidemiological Estimates convened jointly by the UNAIDS Secretariat and WHO on 14-15 November 2007 in Geneva.
The major elements of methodological improvements in 2007 included greater understanding of HIV epidemiology through population-based surveys, extension of sentinel surveillance to more sites in relevant countries, and adjustments to mathematical models because of better understanding of the natural history of untreated HIV infection in low- and middle- income countries. These adjustments to the methodology used are explained in more detail in the box “New data lead to changes in assumptions and improved estimates.” UNAIDS and WHO will continue to modify their estimates of HIV infections and AIDS deaths as new scientific data, research and
analyses emerge.
Several comparisons in this report are made between HIV estimates derived by the uniform revised methodology for 2007 and 2001. The year 2001 was the year of the United Nations General Assembly Special Session on HIV/AIDS that first defined intervention targets, but is also sufficiently long ago to allow meaningful examination of trends in data subjected to uniform analysis.
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