Series
Global epidemiology of HIV infection in men who have sex with men
Introduction
In 2012, men who have sex with men (MSM) are at substantial risk for HIV infection in virtually every context studied (panel 1).1, 3, 4 This risk has been present since the syndrome now known as AIDS was first described in previously healthy homosexual men in Los Angeles (CA, USA) in 1981.5, 6, 7 Despite decades of research and community, medical, and public health efforts, high HIV prevalence and incidence burdens have been reported in MSM throughout the world.8 In many high-income settings—including Australia, France, the UK, and the USA—overall HIV epidemic trends are in decline except in MSM, where they have been expanding in the era of highly active antiretroviral therapy (HAART) in what have been described as re-emergent epidemics in MSM.9, 10 In the USA, HIV infections in MSM are estimated to be increasing at roughly 8% per year since 2001.9 And in much of Africa, Asia, and Latin America, the highest rates of HIV infection in any risk group are in these men.8
However, our understanding of worldwide epidemiology is far from complete. By the end of 2011, 93 of 196 countries had not reported on HIV prevalence in MSM in the previous 5 years.11 In several regions, notably the Middle East, north Africa, and sub-Saharan Africa, data for HIV infections in MSM are only emerging.12, 13 Data gaps and challenges to HIV research, surveillance, and epidemiological characterisation in MSM are largely the result of the hidden and stigmatised nature of MSM populations in much of the world, and of ongoing criminalisation of homosexuality and other forms of same-sex behaviour.11 These structural realities have limited our understanding, and might also have crucial roles in the vulnerability of MSM to HIV.14, 15 We review the global epidemiology and disease burden of HIV infection in MSM; individual-level, couple, and network-level risks for HIV acquisition and transmission; biological aspects of anorectal HIV transmission; and molecular epidemiology advances, with the aim of understanding why MSM continue to bear such disproportionate burdens of HIV. We also developed and report on stochastic agent-based simulation models of HIV transmission to further clarify the drivers of HIV spread in MSM.16 Finally, we discuss the public health importance of our emerging understanding of the epidemiology of HIV in MSM.
Section snippets
Disease burden of HIV in MSM
We did a comprehensive search for HIV burden and risks in MSM from Jan 1, 2007, to June 30, 2011 (search criteria in the appendix). We retrieved 2105 unique citations, and we identified and reviewed 68 additional surveillance studies in the public domain. We included country progress reports submitted to the UN General Assembly Special Session on HIV/AIDS (UNGASS). We obtained data from 82 peer-reviewed publications on disease burden of HIV in MSM, from 12 of the 68 surveillance reports, and
Risks for HIV infection
Individual-level risks for HIV acquisition in MSM have been well documented, and include unprotected receptive anal intercourse, high frequency of male partners, high number of lifetime male partners, injection drug use, high viral load in the index partner, African-American ethnic origin (in the USA), and non-injection-drug use, including use of amphetamine-type stimulants (ATS).33, 34, 35 Recent data suggest individual-level risks might be insufficient to explain the high transmission
Biological factors
The biology of anal sex, the gut tropism of HIV-1,74 and the practices and behaviours associated with anal sex, might at least partly explain the high transmission efficiency of HIV infection for this practice. A recent systematic review and meta-analysis of HIV transmission risks in anal sex24 reported a 1·4% per-act probability (95% CI 0·2–2·5) of transmission for anal sex and a 40·4% per-partner probability (6·0–74·9).24 Per-act probabilities did not differ for MSM or heterosexual anal sex.
Molecular epidemiology of HIV-1 in MSM
Recent reports from molecular epidemiology, phylodynamic studies, and HIV virology are providing insights into transmission and acquisition risks for MSM, transmission dynamics in MSM networks, and challenges to HIV prevention for these men. In a 2008 report on HIV transmission dynamics across the city of London,38 episodic bursts of transmission in large linked clusters were identified as characteristics of transmission within MSM populations.38 About 25% of all HIV infections in MSM were
Modelling HIV risk in MSM
We developed a stochastic agent-based network simulation model of HIV transmission to show the size of some key drivers of HIV epidemics for MSM discussed above (details given in the appendix). These drivers include the high per-act transmission rate for anal sex relative to vaginal sex; the unique ability for MSM to be role versatile within high-transmission acts; and the existence of high numbers of partners within a subset of the population. The model thus shares some goals of previous work
Public health significance
Our findings on the epidemiology of HIV in MSM have many implications for HIV prevention, treatment, and care. The second101 and third102 reports in this Series will address these implications in detail and propose a targeted set of interventions for prevention for these men. First, the high transmission probability, high force of infection, and the potent effect of prevalent HIV infections in networks clearly suggest that interventions to reduce infectiousness, such as HAART for HIV-positive
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