A new Lancet Series published to coincide with the AIDS2012 meeting highlights a population where the HIV epidemic is growing in countries of all incomes: men who have sex with men (MSM). The first paper in the Series charts the epidemiology of HIV among MSM, and is by Professor Chris Beyrer, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA, and colleagues.
The authors conclude that differences between the HIV epidemic in MSM and other populations cannot be explained only by behavioural factors like condom use and multiple partners. The biology of HIV transmission in anal sex, MSM network features, and their related dynamics are key drivers, meaning that reducing infectiousness through initiatives like treatment as prevention and pre-exposure prophylaxis are likely to be key in curbing HIV in MSM.
Some countries reporting data have a HIV prevalence among MSM of 15% or higher: including the USA, Spain, Chile, Thailand, Malaysia, South Africa, and a cluster of other African and Caribbean nations. Italy, Brazil, India, and Canada are among nations with a prevalence of 11-15%. Across most of Western and Northern Europe, the range is 1-6%, including the UK. The region with the highest prevalence of HIV in MSM is the Caribbean (25%) followed by Africa (18%) and North America (15%). Western and Central Europe has a prevalence of 6%.
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 even in this modern era of highly active antiretroviral therapy (HAART). These increasing HIV infection rates have been have been described as re-emergent epidemics in MSM. In the USA, HIV infections in MSM are estimated to be increasing at roughly 8% per year since 2001. And in much of Africa, Asia, and Latin America, the highest rates of HIV infection in any risk group are in these men.
Well-documented risk factors for HIV infection among MSM 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-like substances (ATS). But these risks alone are insufficient to explain the dramatic differences between HIV epidemics among MSM and other populations.
The authors report that the disproportionate HIV disease burden in MSM is explained largely by the high per-act (estimated at 1.4%) and per-partner transmission probability of HIV transmission in receptive anal sex. This is roughly 18-times greater than has been estimated for penile–vaginal intercourse. While for insertive anal sex the risk per act and per partner is far lower than for receptive sex, MSM networks are unique in that role reversal and mixing is common, making it easy for someone to be infected in a receptive role and then infect someone else as the insertive partner. The authors used modelling to estimate that if the HIV transmission probability of receptive anal sex was similar to that associated with unprotected vaginal sex, the 5 year cumulative HIV incidence in MSM would be reduced by 80-98%. Additionally, they estimated that if unprotected anal sex currently occurring in casual partnerships instead occurred in long-term partnerships, new HIV infections could be reduced by 29-51%.
The author also report on the unique feature of HIV epidemics among MSM at molecular levels. They found that rates of infection with multiple HIV strains are much more common in MSM than in heterosexuals, and that HIV viral diversity was markedly greater among these men. These molecular features support the differing epidemiology of HIV among MSM.
The authors say: “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 MSM, will probably be essential to achieving control of these epidemics...Interventions that reduce the probability of acquisition for men engaging in unprotected receptive anal intercourse, such as oral pre-exposure prophylaxis, a rectal microbicide, or successful treatment for prevalent HIV infections, will probably be key.”
They conclude: “Reducing the HIV transmission risk for MSM will probably need combination approaches, the use of antiviral drugs for both treatment and prevention, and much greater understanding of why these men, their networks, and their communities, continue to bear such heavy burdens of HIV. HIV remains uncontrolled in MSM in 2012. This reality demands reinvigorated effort, new approaches grounded in biology and epidemiology, and concerted effort to reduce the structural risks that aid and abet HIV spread in these men.”