The HIV (human immunodeficiency virus) and AIDS epidemic, which emerged in the last quarter of the twentieth century, has within less than two decades spread to over 190 countries in all continents (1). The infection is caused by two main types of virus, HIV-1 and HIV-2. Globally, HIV-1 accounts for the great majority of HIV infections; HIV-2 appears largely confined to West Africa, with foci in Angola and Mozambique and some cases in Europe, the Americas and India (2). Furthermore, HIV infection is characterized by a wide genetic diversity of viral strains in different parts of the world (3). In vitro studies have shown important differences in the biological properties of HIV-1 subtypes (4), but very little is known about possible differences in the transmissibility of these subtypes. In this paper, the abbreviation HIV refers to HIV-1.
Over the last decade AIDS and HIV surveillance has been established by national AIDS programmes in collaboration with the World Health Organization (WHO) in most countries. HIV surveillance and estimates can be used routinely in assessing the magnitude of the problem, in keeping intervention activities focused when estimates are available by subpopulation, and in tracking the development of regional epidemics in the context of the global situation. This paper presents the methods and results of global HIV/AIDS estimates, describes subcontinental characteristics of the epidemic, and discusses important trends which emerge and their possible determinants.
WHO uses several methods for estimating and forecasting, based on empirical data which are drawn from the global disease surveillance system for AIDS and HIV sentinel surveillance, as well as published studies and the WHO/GPA country files which are updated continuously through contact with professionals in the country (5). The process is based on a strong relationship between surveillance, estimation, forecasting and evaluation of trends, both in developing capability at the country level and ensuring that results can be interpreted comparatively and in a global context. A detailed account of the methods has been published elsewhere (1, 5 ). Briefly, WHO bases its estimates on a review of HIV seroprevalence studies, reported AIDS cases, estimates of underreporting, population size and structure (including the age/sex distribution and urban/rural differentials), and the predominant modes of transmission. Two important methods are combined, the HIV estimation process and the use of a fore-casting method for validation, with consistency between HIV incidence, prevalence and AIDS cases and their future projections.
The estimation procedure for country-specific prevalent infections may be described in five basic stages. First, subpopulations in which there is evidence of HIV infection are identified by reviewing all available data, regardless of their quality. Second, prevalence studies are reviewed for their methodological qualities according to criteria described previously (5), and studies with a predetermined threshold sample size are selected. Third, using all remaining seroprevalence data points to provide upper and lower bounds, a conservative estimate (i.e., usually lower than the median value of all prevalence levels) is selected for each subpopulation, taking into account seroprevalence trends over the past two years. Fourth, the best available information is used to estimate the size of the subpopulation. Finally, the estimated prevalence rates are applied to the estimated subpopulations' sizes and totalled to provide an estimate of prevalent infections for the country (5).
Once a provisional country-specific estimate is obtained, the reported and estimated AIDS cases are compared with the numbers that could be expected from estimated past and present HIV levels, using a projection model (usually, but not always, epimodel (6)) with the appropriate inputs, and the estimate is adjusted accordingly (1). …