The prevalence of HIV among the population 15-49 years old is measured as the number of individuals aged 15-49 living with HIV divided by the total population aged 15-49.
UNAIDS and WHO, in close consultation with countries, developed a methodology to obtain national estimates of HIV prevalence for men and women. Different approaches are used for generalized epidemics (where adult HIV prevalence exceeds 1% and transmission is mostly heterosexual) and low-level or concentrated epidemics (where HIV is concentrated in groups with behaviours that expose them to a high risk of HIV infection). For countries with generalized epidemic:
All available surveillance data gathered from HIV-tested blood samples of pregnant women attending antenatal clinics and HIV prevalence results from population-based surveys are entered into a specialized software programme (called the “Estimation and Projection Package”). This programme generates a curve that describes the evolution of adult HIV prevalence rates over time. This adult prevalence curve, along with the national population estimates obtained from the UN Population Division, the antiretroviral therapy (ART) coverage, and various epidemiological assumptions (fertility rates, male/female ratios, survival time after HIV infection, etc.) are then entered into the Spectrum software programme which calculates the number of adults and children infected, new infections, deaths, orphans and treatment needs.
For countries with a low-level or concentrated epidemics:
Surveillance data are gathered for populations at high risk (such as sex workers, men who have sex with men and injecting drug users). Estimates are made of the size of those populations, and of populations that are at lower but significant risk (such as the partners of sex workers and their clients, injecting drug users, etc.). That information is then entered into the “Estimation and Projection Package” to produce curves that describe the evolution of adult HIV prevalence rates over time. This adult prevalence curve, along with the national population estimates obtained from the UN Population Division, the ART coverage, and various epidemiological assumptions (fertility rates, male/female ratios, survival time after HIV infection, etc.) are then entered into the Spectrum software programme which calculates the number of adults and children infected, new infections, deaths and treatment needs.
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Improved methods, enhanced data and new estimation tools are enabling a better understanding of the degrees of uncertainty that surround HIV and AIDS estimates. This is part of an ongoing process of improving estimates and developing appropriate ranges—all of which are vital for effective HIV/AIDS planning and programming at national and regional levels.
Because the quality of data varies from country to country, the ranges of uncertainty surrounding estimates can widen or narrow depending on the country. The ranges reflect the degree of uncertainty associated with estimates and define the boundaries within which the actual numbers lie.
Four factors determine the extent of the ranges around the HIV estimates:
(i) The HIV prevalence level – Ranges tend to be smaller when HIV prevalence is higher. Thus the bounds around the best estimate of adults living with HIV in Zambia are relatively small (1,100,000 – 1,200,000) while they are much wider in a lower prevalence country such as Djibouti (3,900 – 31,000).
(ii) The quality of the data – Countries with better quality data have smaller ranges than countries with poorer quality data. The ranges for Asia and the Pacific are comparatively broad—which reflects the fact that HIV surveillance of key populations (such as injecting drug users, sex workers and men who have sex with men) is relatively poor in most countries in that region, hence resulting in more uncertainty. In general, the ranges for sub-Saharan Africa are narrower, because of recent improvements in the collection and interpretation of HIV data in that region (including the availability of a national survey for most countries).
(iii) The number of steps or assumptions used to arrive at an estimate – The more steps and assumptions, the wider the uncertainty range is likely to be (since each step introduces additional uncertainties). For example, ranges around estimates of adult HIV prevalence are smaller than those around estimates of HIV incidence among children, which require additional data on the probability of mother-to-child HIV transmission. The latter are based on prevalence among pregnant women, the probability of mother-to-child HIV transmission, and estimated survival times for HIV-positive children. There is therefore greater uncertainty in these estimates than for adult prevalence alone.
(iv) The type of epidemic (generalized or low-level/concentrated)– Ranges tend to be wider in countries with low-level or concentrated epidemics than in countries with generalized epidemics because in low-level or concentrated epidemics, one needs to estimate both the numbers of people in the groups at higher risk of HIV infection and HIV prevalence rates.
Assumptions, methodologies and data used to produce the estimates are gradually changing as a result of ongoing enhancement of our knowledge of the epidemic; hence comparisons of recent estimates with those published in previous years is liable to yield misleading conclusions.