Response Monitoring and Analysis Unit
Evidences, Strategy and Results Department
Joint United Nations Programme on HIV/AIDS (UNAIDS)
Postal Address:20 Avenue Appia, 1211 Geneva 27, Switzerland
Telephone: +41 (0)22 791 2927
Fax: +41 (0)22 791 4798
Epidemiology and Analysis Division
Telephone: +41 (0)22 791 4601
Fax: +41 (0)22 791 4251, +41 (0)22 791 4812
The incidence rate is the number of new HIV infections in a population during a certain time period. People who were infected before that time period are not included in the total, even if they are still alive. Here it is expressed as a percentage of the adult population aged 15-49 years. For example in Sub-Saharan Africa the incidence rate in 2009 was 0.4% meaning that 4 persons out of 1000 got newly infected during 2009.
Human Immunodeficiency Virus (HIV) is a virus that weakens the immune system, ultimately leading to AIDS, the acquired immunodeficiency syndrome. HIV destroys the body’s ability to fight off infection and disease, which can ultimately lead to death.
To calculate the adult HIV incidence, the estimated number of adults (15-49 years) newly infected with HIV in a given year is divided by the adult population (15-49 years) not infected at the start of the same year.
Unfortunately, directly measuring HIV incidence is a complex process, therefore national, regional and global estimates of HIV incidence are usually produced by computer models and are based on estimates of HIV prevalence. The general methodology and tools used to produce the country-specific estimates including incidence rates have been described in a series of papers in Sexually Transmitted Infections 2010: “Methods and tools for the 2009 HIV and AIDS estimates and projections, and related analyses 86 (Suppl 2)”.The estimates produced by UNAIDS/WHO are based on methods and parameters that are informed by the UNAIDS Reference Group on HIV/AIDS Estimates, Modelling and Projections, described in reports available at www.epidem.org . This group is made up of leading researchers in HIV and AIDS, epidemiology, demography and related areas. The Reference Group assesses the most recent published and unpublished work drawn from research studies in different countries. It also reviews advances in the understanding of HIV epidemics and suggests methods to improve the quality and accuracy of the estimates.
The incidence rate data is presented together with ranges, called ‘plausible bounds’. These bounds reflect the certainty associated with each of the estimates. The wider the bounds are, the greater the uncertainty surrounding an estimate. The extent of uncertainty depends mainly on the type of epidemic, 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.
For additional details see:
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.
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.
The global estimates are representative of the national estimates for a given year.
Regional workshops are conducted every 2 years to produce draft estimates. These are finalized through correspondence with the country.
The UNAIDS Epidemiology team collaborates with national counterparts to generate HIV estimates for their country. Typically, the coordinating counterparts are based in the National AIDS Council or in the Ministry of Health, and they receive inputs from the AIDS programme (e.g. ART, Preventing Mother To Child Transmission programmes) and from the Statistics office.
Country estimates are collected and reviewed based on new findings at the country level, as well as previous data trends. Country data is validated by country representatives for accuracy.
No adjustments are made for international comparability. The data are comparable because of the well-standardised methodologies. HIV estimate data are reported as rounded figures.
There is no treatment of missing values. When the information needed to calculate the indicator is not available, the indicator is not estimated.
Incidence data (estimations) are available for all regions and from 60 countries, The number of countries with estimated incidence data is continuously increasing.
Incidence is estimated for people aged 15-49.
The lag between the reference year and actual production is from the period of regional workshops conducted every 2 years to the publication of the data.
Regional and global estimates are estimated using the epidemiological tools described in the following website:
The mid point estimates, upper bounds and lower bounds of the HIV global incidence, prevalence and AIDS deaths indicators, used for the production of the 2010 MDG report, can be seen here.
Updated incidence data is released every two years.