Adolescents and youth aged 15-24 are one of the populations most impacted by the global HIV epidemic with an estimated 50% of new infections occurring in this age group. They are thus one of the prime populations for targeting behavioral and biomedical preventions. However, the dynamics of the HIV epidemic in youth vary widely by geographic region and risk behavior profiles. There are also biological and neurodevelopmental considerations that must be considered in the development, testing, and ultimate dissemination of HIV prevention interventions. These concepts are broadly discussed here.
From the *Department of Epidemiology, University of Alabama at Birmingham, Birmingham AL; †Department of Pediatrics, Dartmouth College, Hanover, NH; ‡Elizabeth Glaser Pediatric AIDS Foundation, Santa Monica, CA; and §Department of Pediatrics, New York University School of Medicine, New York, NY.
This article was based on a presentation made at the Consultation for the Inclusion of Adolescents in Biomedical HIV Prevention Clinical Trials, June 17-19, 2009, Washington, DC.
Sources of support: The development of this article was not supported by any public or private funding sources.
Correspondence to: Craig M. Wilson, MD, Department of Epidemiology University of Alabama at Birmingham School of Public Health RPHB 437, 1530 3rd Avenue S Birmingham, AL 35294-0022 (e-mail: firstname.lastname@example.org).
It is widely known that adolescents and young adults are at or near the epicenter of the global HIV epidemic across almost all geographic risk profiles and locations. This article is meant to highlight some of the features of the global HIV epidemic in youth and set the background for discussions of prevention interventions targeting this population. Characteristics of various global regions or risk profiles among youth that will impact development, testing, and ultimately dissemination of HIV prevention interventions will be described.
The definition of adolescence varies depending on the organization and the type of report being produced. For example, the Centers for Disease Control and Prevention often refer to adolescents (10-19) and young adults (20-24) when discussing this transition period between childhood and adulthood.1 The World Health Organization often refers to young people and includes individuals from 10 to 24 years of age.2-3 Although this grouping is convenient for epidemiological purposes, there clearly are developmental, biological, and legal reasons for segmenting these populations when considering development of behavioral or biomedical prevention interventions and their testing. These types of issues will be discussed by other authors in appropriate sections of this collection.
As of 2007, there were 1.2 billion young people aged 15-24 in the world with an estimated 10 million living with HIV. Current estimations by Joint United Nations Program on HIV/AIDS suggest that more than 1 million new HIV infections occur in the 15-24 age group each year, representing more than 40% of worldwide new infections.4 Of the 15-year to 24-year olds living with HIV, 63% live in sub-Saharan Africa and 21% live in Asia Pacific. In Eastern Europe and Central Asia, more than 80% of those living with HIV are younger than 30 years. Sub-Saharan Africa contains almost two-thirds of all young people living with HIV/AIDS (6.2 million) with 76% of them being female.4
EPIDEMIC BY RISK BEHAVIORS
Different risk behaviors are driving the epidemic depending on the area of the world under consideration. However, a common theme throughout the differing epidemics in different parts of the world are the impact of individual-level risks as the core factors determining transmission and the many social network, community, and public policy factors potentially impacting these individual-level risks. An example of the listing of these various levels of risk that make up an ecological model for Southern Africa is shown in Table 1.
Heterosexual transmission is driving the epidemic in Africa, especially affecting young women of child-bearing age. For adolescent “females,” the risk of HIV acquisition is directly related to the age and risk profile of their partners. Thus, in areas where “heterosexual transmission” propels the local epidemic as in sub-Saharan Africa, females are becoming infected at younger ages than males through exposure to older males. Data from several surveys from South Africa clearly illustrate such trends.5-7 Recently, concurrency8 and circumcision9 have been discussed as key elements driving the heterosexual HIV epidemic in sub-Saharan Africa, but other factors must be considered as prevention strategies are being designed, tested, and ultimately disseminated. As in sub-Saharan Africa, in other areas such as the Caribbean, Guyana, and some Central American countries, heterosexual transmission is a dominant mode of transmission, and similar patterns of acquisition are seen.4 In general, in these areas where heterosexual transmission of HIV is predominant, “males” are becoming infected at slightly higher ages than females and at lower rates. This is attributed to males having either younger or more similar aged female partners at coital debut.6-7
The HIV epidemics in North and Latin America, Central and Western Europe, and Oceania are predominantly men who have sex with men (MSM)-driven epidemics.4,10-12 There is evidence in the United States that the HIV epidemic among young MSM, particularly ethnic minorities, is the most rapidly expanding segment of the overall US epidemic.13-15 Similarly, there is evidence in many other regions of an expanding MSM epidemic even in the setting of decreasing overall HIV rates.4,10,16-18 The individual-level risk for MSM is unprotected anal intercourse, particularly anal-receptive intercourse. Within the MSM population, transgender youth have particularly high rates of HIV incidence. The ecological context for the MSM HIV epidemic in many areas is particularly confounded and challenged by policy and community factors, that is, homosexuality is illegal, highly stigmatized, or not even recognized as a phenomenon.19 It is not surprising then that data on MSM, particularly young MSM, are lacking in many geographical settings. Similarly, the interplay of MSM and heterosexual HIV transmission are not clear in most settings.
HIV transmission to youth from intravenous drug use (IVDU) is the predominant mode of acquisition in most Eastern European and some Asian settings.20 Although IVDU transmission does occur in many other settings, it is usually in an older population. IVDU is quite commonly the initial recreational drug exposure in Eastern Europe leading to this epidemic among youth.20,21
As scientists research and eventually implement HIV prevention programs, it is important not only to consider the differences based on location but also differences across different neurodevelopmental strata of youth. Neurocognitive differences between early adolescence (aged 11-14), middle adolescence (aged 15-17), late adolescence (aged 18-21), and youth (aged 21-24) must be taken into account when evaluating potential interventions. Other biological changes with pubertal development including changes in body mass, immune function, and sexual maturation will be having significant effect on the efficacy and safety of specific interventions and should be evaluated during development and considered during ultimate deployment of biomedical interventions.
This overview of HIV epidemiology among youth is meant to establish the context for a discussion of prevention interventions targeting youth. These data will need to be updated and refined as this epidemic evolves in its many settings. Further, the more dissected ecology of the local HIV epidemic will need to be considered for testing prevention interventions and will be crucial for ultimate dissemination of the multifaceted interventions it will take to impact the global HIV epidemic.
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