Prevalence/Incidence - Definition in HIV/AIDS

HIV infection, artwork
HIV infection, artwork. Getty Images/Science Photo Library/Brand X Pictures


Prevalence is a term used in epidemiology to describe the proportion of a population identified as having a certain condition (e.g. HIV/AIDS). The prevalence figure is determined by comparing the number of people found to have the condition with the total number of people in that population group (e.g., proportion of African Americans with HIV residing in New York City). Prevalence is most often described as a percentage (%).

In HIV, prevalence is used by public health officials and policy makers to identify the burden of HIV infection on certain regions and/or population groups. The population groups may be stratified by ethnicity, gender, age, sexual orientation, economic status, drug use culture, or a combination of any or all of these categories.

Incidence measures the risk of a condition developing during a specific period of time. The figure is arrived at by comparing the number of new cases reported during a certain period with the total number of people in that population. The figure can either describe a proportion (e.g. 45 cases out of 1,000 people) or a percentage (4.5%).

In HIV, the incidence is often used to estimate whether the risk of HIV (or an HIV-related illness) is increasing or decreasing within a certain group, usually on a year-on-year basis. Incidence is also used by researchers to determine if a change in a certain factor (e.g. drug therapy, public policy) may alter risk within population groups.

In addition, predicting risk through incidence analysis allows for optimal resource allocation.

At their most basic, prevalence describes the "here and now," while incidence describes what will likely be.

Examples: 5,600,000 people in South Africa were estimated to be infected with HIV in 2009. With a total population of 53 million, the HIV prevalence is South Africa is said to be 10.6%.

When looking specifically at adults aged 15 to 49—considered the age group most at risk for HIV infection worldwide—the prevalence increases to 17.3% (the figure used by the World Health Organization for comparative national surveys).

In 2006, HIV incidence among men who have sex with men (MSM) in San Francisco was 1.75%, based on 772 new infections in a population of 44,138 HIV-negative MSM. With the introduction of aggressive, new public health policies in 2010 (which included universal treatment on diagnosis), the incidence in dropped to 1.27% by 2011. With MSM rates climbing in most major U.S. cities, the change in incidence was seen to be significant and indicative the effectiveness of the new policies.


United Nations Joint Programme on HIV/AIDS (UNAIDS). "Epidemiological Fact Sheet - South Africa." Geneva, Switzerland; 2009.

McFarland, W. "HIV/AIDS Epi-Update for San Francisco - The Numbers." San Francisco Department of Public Health. November 29, 2006; Powerpoint presentation.

Bajko, M. "HIV continues to retreat in SF." Bay Area Reporter. Published March 24, 2011.

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