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What are the different epidemiological scenarios?

Key to planning an effective HIV prevention response is knowing who is at higher risk of HIV; the extent to which HIV is prevalent amongst different populations; and the risk behaviours, laws, and policies that may facilitate the transmission of HIV. For the purpose of epidemiological surveillance, UNAIDS and WHO categorize epidemiological scenarios as:

In low-level scenarios, HIV has not spread to significant levels in any sub-population. The low-level epidemic suggests either that networks of risk are diffuse (with low levels of partner exchange or use of non-sterile injecting equipment), or that the virus has been introduced only very recently. In low-level epidemic situations, basic information about the most vulnerable and at risk populations is needed and must be collected in an ethically sound manner (1). Empirical study of risk behaviours, networks and other factors indicating the potential for HIV spread, such as rates of other sexually transmitted infections is essential for prevention planning.

(1) UNAIDS (2006). Monitoring and Evaluation of HIV prevention Programmes for Most-at-Risk Populations. A Framework for Monitoring and Evaluation HIV Prevention Programmes for Most-at-Risk Populations. Joint United Nations Programme on HIV/AIDS, Geneva.

In concentrated scenarios, HIV prevalence is high enough in one or more sub-populations, such as men who have sex with men, injecting drug users, or sex workers and their clients to maintain the epidemic in that sub-population (1), but the virus is not circulating in the general population. The future course of an epidemic of this type will be determined by the size of the vulnerable sub-population[s] and the frequency and nature of links between sub-populations and the general population and the degree of responding to the needs of the affected and most vulnerable populations. Spread of HIV can be explosive in settings with injecting drug use (2).

(1) Anderson R. May R (1991). Infectious Diseases of Humans: Dynamics and Control, Oxford University Press, Oxford.
(2) IOM (2006). Preventing HIV Infection among Injecting Drug Users in High Risk Countries: An Assessment of the Evidence.
IOM of the National Academy of Sciences, Washington, DC.

A number of countries now consistently report an HIV prevalence of between 1–5% in pregnant women attending antenatal clinics, indicating that the presence of HIV among the general population is sufficient for sexual networking to drive the epidemic. In these epidemic scenarios, HIV transmission in serodiscordant couples and multiple partner relationships that give rise to sexual networks in the general population — account for the majority of new infections. Most-at-risk populations such as sex workers and their clients are still at risk of HIV infection. However, the behaviours of very large sub-populations, with relatively low risk (such as unmarried young people, and married women and men who do not regularly visit sex workers or have multiple partners) contribute to the larger proportion of new infections. In a generalized epidemic with more than 5% adult prevalence, no sexually active person is “low risk”.

In some parts of Africa, there is research evidence on sexual networks which shows that longer-term multiple concurrent partnerships intensify the epidemic (1). Broad social norms that lead to multiple sexual partner relations and/or norms and policies that prevent individuals or populations from protecting themselves (for example gender norms that lessen girls’ access to education and information), are directly implicated in the epidemic dynamics. As more women are living with HIV, a significant number of new infections are from mother-to-child transmission (2).

(1) Morris M, Levine R, Weaver M (2004). Sexual networks and HIV Programme Design. The Synergy Project, Washington, DC.
(2) WHO/UNFPA (2006). Glion Consultation on Strengthening the Linkages between Reproductive Health and HIV/AIDS: Family Planning and HIV/AIDS in Women and Children. World Health Organization, Geneva.

This is a situation in which HIV is established in the general population, yet differences in both the level and the drivers and risk factors of the epidemic require additional strategies for effective HIV prevention. HIV has spread to a level above 15% in the adult population, through extensive heterosexual multiple concurrent partner relations with low and inconsistent condom use. All sexually active persons have an elevated risk of HIV infection. The drivers and risk factors of this predominantly heterosexual epidemic are complex and diverse, but may include behaviours such as early sexual debut, high levels of longer-term multiple concurrent sexual partnerships—especially for men, inter-generational sex, gaps in consistent condom use with casual and longer-term partners, low acceptability of condom use in cohabiting couples and biological co-factors such as low levels of male circumcision and the presence of sexually transmitted infections especially viral infections which are difficult to treat (1). High levels of HIV-related stigma, gender based violence, including sexual coercion and violence in marriage, gender inequality and geographic mobility result in rapid and continuing spread of HIV in the general population, leading to and maintaining very high prevalence. Stigma may also lead many individuals to avoid risk reduction behaviours (e.g. abstinence, partner limitation, disclosure of status to sexual partners if known, correct condom use) because of the association of these behaviours with being HIV positive. It is important to remember that there is still significant geographic variation in HIV prevalence within these countries.

(1) SADC (2006). Expert Think Tank Meeting on HIV Prevention in High Prevalence Countries in Southern Africa Report. Southern African Development Community, Maseru.