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HIV most often passed on by younger gay men who have undiagnosed HIV, are not on treatment, and have an ongoing partnership
Roger Pebody, 2014-12-11 10:20:00

A modelling study based on the UK’s HIV epidemic among gay men estimates that two-thirds of infections originate in men with undiagnosed HIV, 85% in men who are not taking treatment and 90% within the context of an ongoing sexual partnership. Moreover, HIV transmissions most frequently involve men under the age of 35 who report relatively high levels of sexual activity, according to the study published online ahead of print in AIDS.

But the findings suggest that men with primary infection contribute less to onward transmission than is sometimes thought.

The model aimed to simulate the underlying drivers of the epidemic – the contribution of various behavioural and biological factors to HIV transmission between men who have sex with men (MSM).

A better understanding of who is passing HIV on could help with the design and targeting of HIV prevention and testing interventions. It could also help explain why – despite the high uptake of HIV treatment – there are no signs of a fall in new HIV infections among gay men in the UK.

The researchers fed into their model data on HIV diagnoses, uptake of HIV treatment, CD4 counts, the sexual behaviour of UK gay men (condom use, rate of partner change, serosorting, etc.), as well as the risk of HIV transmission during various sexual behaviours and at various viral loads.

The key figures produced are known to statisticians as ‘population attributable fractions’ – this estimates the proportion of HIV transmissions that originate in a particular group of people. For example, the method can estimate the proportion of transmissions that come from people with diagnosed and undiagnosed HIV infection. As there is some fuzziness in some (but not all) of these estimations, the researchers also report a range of credible figures – the true figure is likely to be somewhere between the two.