There is interest in expanding ART to prevent HIV transmission, but in the group with the highest levels of ART use, men who have sex with men (MSM), numbers of new infections have not decreased as coverage has increased for reasons which remain unclear. We analysed data on the HIV epidemic in MSM in the UK from a range of surveillance sources using a individual-based simulation model, allowing us to reconstruct the epidemic’s main features. Model runs using parameter sets found to result in good model fit were used to infer changes in HIV-incidence and risk behaviour. Our results indicate that HIV incidence has increased (estimated mean incidence 0.30/100 person-years 1990-1997, 0.45/100 py 1998-2010), associated with a modest (26%) rise in condomless sex. Our model allows us to explore counter-factual scenarios: had ART not been introduced, but the rise in condomless sex had still occurred, then incidence 2006-2010 would have been 68% higher (95% CI 62% to 74%); a policy of ART initiation in all diagnosed with HIV from 2001 would have resulted in 32% lower incidence (95% CI 27% to 37%); had levels of HIV testing been higher (68% tested/year instead of 25%) incidence would have been an estimated 25% lower; a combination of higher testing and ART at diagnosis would have resulted in 62% lower incidence; cessation of all condom use in 2000 was predicted to result in a 424% increase in incidence. We conclude that a rise in HIV-incidence has occurred in MSM in the UK despite an only modest increase in levels of condomless sex in the era of ART. ART has almost certainly exerted a limiting effect on incidence. Much higher rates of HIV testing combined with initiation of ART at diagnosis would be likely to lead to substantial reductions in HIV. However, before advocating such a policy for all MSM we await data from well-powered randomized trials of the individual health risk of ART initiation at CD4>350 cells/mm3, and from studies of the effect of ART on infectivity through anal sex. Increased condom use should be promoted to avoid the erosion of the benefits of ART and to prevent other serious sexually transmitted infections.
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