Understanding HIV Prevalence and Incidence

Prevalence is the proportion of a population identified as having a certain condition. The prevalence figure is determined by comparing the number of people found to have a condition with the total number of people in that population group. Prevalence is most often reported as a percentage.

Prevalence is among the HIV statistics used by public health officials and policymakers to identify the burden of human immunodeficiency virus (HIV) infection in certain regions and/or population groups. The population groups may be classified by ethnicity, gender, age, sexual orientation, economic status, drug use, culture, or a combination of any of these categories.

HIV Prevalence map in the United States, 2010
AIDSVu / Emory University

The Difference Between Prevalence and Incidence

Prevalence and incidence can be related to each other, but they aren't exactly the same. Incidence is the number of new cases, while prevalence is the total number of existing cases. At their most basic, prevalence describes the here and now, while incidence can be used to try to predict what will likely occur.

The incidence 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 be reported as a proportion or a percentage.

Incidence is often used to assess whether the risk of HIV has been increasing or decreasing within a certain group, usually on a year-on-year basis. Trends in incidence are also used by researchers to determine whether a change in a certain factor—such as access to treatment or changes in public policy—may be able to alter the risk within population groups. Predicting risk through incidence analysis helps guide optimal resource allocation.

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Understanding HIV and AIDS

Examples

The prevalence can differ among different populations within a specific region. Researchers discovered that 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 in South Africa was 10.6 percent.
  • When looking specifically at adults aged 15 to 49—considered the age group most at risk for HIV infection worldwide—the prevalence was 17.3 percent.

Incidence can change over a short period of time.

  • The HIV incidence among men who have sex with men (MSM) in San Francisco in 2006 was 1.75 percent, based on 772 new infections in a population of 44,138 HIV-negative MSM.
  • With the introduction of new public health policies in 2010, including universal treatment at the time of diagnosis, the incidence dropped to 1.27 percent by 2011.

With MSM rates of HIV infection climbing in most major U.S. cities, this drop in incidence was seen to be indicative of the effectiveness of the new policies.

U.S. Prevalence and Incidence

From a global perspective, while the prevalence and incidence of HIV can vary from country to country, there is typically an association between disease burden and how effectively a country is managing the epidemic within its borders.

For example, while the estimated 1.2 million infections in the United States may pale in comparison to numbers seen in some areas of the world, the prevalence and incidence of the disease paint a starkly different picture. When compared to most other developed countries, the United States has the highest prevalence (0.6%) and incidence (15.3 per 100,000). 

By contrast, the prevalence rate in most high-income countries falls well below 0.3%, while the median incidence rate is less than half that seen in the United States (6.3 per 100,000).

It is hoped that recent developments in policy and advancements in preventative medications may reduce the prevalence and incidence of HIV infection within the United States. However, low rates of care and retention among the most vulnerable populations, such as African Americans and MSM, could continue to fuel new infections.

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