Prévalences du VIH en Afrique sub-saharienne : Historique d’une estimation

Abstract

Tous les deux ans, le programme commun des Nations-Unies sur le VIH/SIDA (ONUSIDA) publie des estimations détaillées du nombre de personnes infectées par le VIH (virus de l’immunodéficience humaine). Selon la dernière édition de son rapport, publiée en juin 2008, 33 millions de personnes (adultes et enfants) dans le monde dont 22 en Afrique subsaharienne seraient infectées en 2007. Cette estimation, largement reprise par les médias et les acteurs de la lutte contre le SIDA, a été révisée à maintes reprises ces dernières années. À titre d’exemple, le rapport 2002 estimait ce nombre à 28,5 millions en 2001 pour l’Afrique subsaharienne. Cette même estimation a été révisée à la baisse, dans la dernière édition du rapport, à 20,4 millions en 2001. Paradoxalement, le nombre de personnes infectées a augmenté entre 2001 et 2007. Les révisions successives effectuées par l’ONUSIDA ont semé le doute sur la qualité des estimations produites. Petit retour sur l’évolution des sources et des techniques.

In 2008 UNAIDS global report on AIDS, the number of people living with HIV in sub-Saharan Africa was estimated at 22 millions in 2007 and 20.4 millions in 2001, while in the 2002 report, the same estimation for 2001 was 28.5 millions. Changes in UNAIDS reflects evolutions of data sources and methods used for the estimates. Sentinel surveillance of pregnant women attending antenatal clinics (ANC) was developed in 1980’s with the WHO recommendation of unlinked anonymous testing approach. The objective was not to be representative but to monitor trends. In the 1990’s, as ANC data were available, they were used by EpiModel, a model developed by WHO for HIV prevalence estimates from 1992 to 2000. In 2002, a new epidemiological model called EPP (Estimation and Projection Package) was developed by the UNAIDS Reference Group on Estimates, Modelling and Projections, which, in countries with a generalised epidemic, is still based on ANC data collected over time. Since 2001, many countries have conducted national population-based surveys (NPS). Their results have often diverged from estimates based on ANC data. This was explained by the underrepresentation of rural clinics in sentinel surveillance and relative small participation rate in NPS. Since, several studies have shown that the impact of several biases (participation rate, non-household members, serological window of tests…) in NPS remains relatively low. NPS constitute a good indicator of HIV prevalence level. If pregnant women can be locally representative of the general population, at the national scale, it depends of the localization of selected clinics. But ANC provides data over time, which is not the case for NPS. The current approach of UNAIDS consists in estimating HIV prevalence trends from ANC and the level of the epidemics from NPS. But the hypothesis that ANC data are representative of trends still needs to be verified when several NPS will be available for a same country.