identify biological factors associated with HIV transmission in men who have sex with men.

The objective of the study was to identify biological factors associated with HIV transmission in men who have sex with men. The authors undertook a longitudinal phylogenetic analysis of HIV-1 from a cohort of men who have sex with men, incorporating clinical and epidemiological data. Potential individuals were HIV-infected men who have sex with men attending a sexual health clinic between 2000 and 2006. Individuals were classified such that they could move from recent to chronic infection categories. HIV-1pol gene sequences were obtained from plasma virus or proviral DNA and clusters estimated by maximum likelihood and conservative genetic distance differences. The single most likely transmitter generating each recent infection was ascertained and risk factors around time of likely transmission explored using Poisson regression modelling. Out of 1144 HIV-infected men who have sex with men, pol sequence data were obtained for 859(75%); 159 out of 859 (19%) were recently HIV infected at diagnosis. A single most likely transmitter was identified for 41 out of 159 (26%), of which 11 were recently infected (27%) and 30 chronically infected. Factors associated with transmission in multivariable analysis were: younger age {rate ratio per 5 years older 0.68 [95% confidence interval (CI) 0.54-0.86], P=0.0009}, higher viral load [rate ratio per log higher 1.61 (95% CI 1.15-2.25), P=0.005], recent infection [rate ratio 3.88 (95% CI 1.76-8.55), P=0.0008] and recent sexually transmitted disease [rate ratio 5.32 (95% CI 2.51-11.29), P=0.0001]. Antiretroviral therapy was highly protective in a univariable model, RR 0.14 (95% CI 0.07-0.27, P=0.0001). Onward transmission of HIV among men who have sex with men is significantly associated with recent infection, sexually transmitted diseases and higher viral load, and reduced by effective antiretroviral therapy. The majority of new infections appear to occur from individuals whose Despite the fact that participants in this study were from a geographically-defined cohort where 88% of men who have sex with men obtain care for HIV and sexually transmitted diseases from the same clinic, a likely transmitter could be identified for only 26% of recently infected men. Sophisticated phylogenetic clustering techniques were used to match individuals likely to be part of a transmission chain. Of the 41 men who transmitted HIV, 9 were undergoing a treatment interruption and 2 had undetectable viral load at the likely time of transmission. These are points to raise when counselling HIV-positive men about adherence to antiretroviral treatment and the wisdom of safer sex practices regardless of viral load. For HIV-negative men who have sex with men, as for men who have sex with women or transgender people, consistent condom use and early detection of HIV infection reduce the risk of acquiring HIV in the first place and unknowingly transmitting it to others.