HIV Incidence rate

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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
What it measures

To assess progress towards reducing HIV infection

Rationale
Numerator
Denominator
Calculation

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.

Method of measurement
Measurement frequency
Disaggregation
Explanation of the numerator
Explanation of the denominator
Strengths and weaknesses

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 The Reference Group assesses the most recent published and unpublished work drawn from research studies in different countries.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.

Further information