HIV and AIDS has become a major public health problem in many countries and monitoring the course of the epidemic and impact of interventions is crucial. Both the Sustainable Development Goals (SDGs) and the 2021 United Nations Political Declaration have set goals of reducing HIV prevalence.

The estimated number of adults aged 15-49 years with HIV infection, whether or not they have developed symptoms of AIDS, expressed as per cent of total population in that age group.

• Gender (female, male, other*)

• Age (15–49 years)

• Key populations (men who have sex with men, people living in prisons and other closed settings, people who inject drugs, sex workers, trans and gender diverse people)

• Pregnant women or breastfeeding women

• Cities and other administrative regions of epidemiologic importance

*The category of other includes trans and gender diverse people who choose an identity other than male or female.

Standardized tools and methods of estimation have been developed by UNAIDS and WHO in collaboration with the UNAIDS Reference Group on Estimation, Modelling and Projections. In countries with a generalized epidemic, national estimates of HIV prevalence are based on data generated by surveillance systems that focus on pregnant women who attend a selected number of sentinel antenatal clinics, and in an increasing number of countries on nationally representative serosurveys. In countries with a low level or concentrated epidemic national estimates of HIV prevalence are primarily based on surveillance data collected from populations at high risk (sex workers, men who have sex with men, injecting drug users) and estimates of the size of populations at high and low risk. These data are entered into the Estimation and Projection Package (EPP) software which fits a simple epidemiological model to the epidemic structure defined. EPP finds the best fitting curve that describes the evolution of adult HIV prevalence over time, and calibrates that curve based on prevalence found in any national surveys or default values in case there is no national survey available. For countries with very little available prevalence data (less than three consistent surveillance sites) a point prevalence estimate and projection is made using spreadsheet models (the Workbook Method). The resulting point prevalence estimates for several years are entered into EPP to find the best fitting curve that describes the evolution of adult HIV prevalence over time.

Modelling, using multiple inputs specific to the HIV epidemic context, is typically used to obtain an estimate of the HIV prevalence rate. UNAIDS and WHO support most countries to produce estimates of HIV prevalence annually using Spectrum.

Regional estimates are weighted averages of the country data, using the number of population aged 15–49 years for the reference year in each country as the weight. No figures are reported if less than 50 per cent of the population aged 15–49 years in the region are covered.

Percent

Annual

Annual

The HIV estimates are presented together with ranges, which reflect the certainty associated with each of the estimates. The extent of uncertainty depends mainly on the type of epidemic, and the quality, coverage and consistency of a country’s surveillance system and, in generalized epidemics, whether or not a population-based survey with HIV testing was conducted. The main indicator proposed for monitoring progress towards achieving the international goals is HIV prevalence among young people aged 15-24 years, which is a better proxy for monitoring HIV incidence. Although countries are moving towards collecting better data on young people, mainly by capturing data on young pregnant women attending antenatal clinics or national population based surveys, comparable data availability is still limited. Analysis of trends on consistent sites have been proposed as an alternative tool to assess recent trends and countries have been encouraged to report HIV surveillance data by age breakdown.

beusenbergm@who.int

Michel Beusenberg

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