1 in 5 "straight" men caught their HIV from another man study says

A large-scale genetic analysis shows that a minimum figure of 19% of men with HIV who declare themselves to be exclusively heterosexual in fact have sex with men in the UK. Such individuals are what the researchers call “undisclosed men who have sex with men (MSM)”. The second conclusion of the study was that "undisclosed MSM" were more likely to be the linking partner between a "disclosed MSM" and a woman.

The phylogenetic analysis looked at the 50,025 genetic sequences of HIV subtypes A1, B and C from the UK HIV Drug Resistance Database, which contains sequences from 60% of all people with HIV in the UK. Another 10,000 sequences belonging to other subtypes of HIV were not included.

The "undisclosed MSM" were considerably more likely to be of black African ethnicity rather than any other ethnic background. They were also very much less likely to have had a recent HIV infection. This could mean two things, both of which could be true: "undisclosed MSM" are much more likely to be late presenters, and may also be older than "declared MSM".

This analysis only measures supposedly heterosexual men who only had male contacts. However, there has been a concern since the epidemic began that bisexual men, whether open or ‘closet’, could be links between the gay population and the general population. This has proven not to be a major issue in countries with concentrated epidemics but can become a factor in countries with more generalised ones.

The researchers, therefore, did an analysis of the relatively few clusters of three or more people in the network where “bridging” between declared MSM and heterosexual women or men did happen. They discovered two types of linkage. In one, the woman was a link between an openly bisexual MSM and a heterosexual man, i.e. she received her infection from one and passed it on to the other. In the second, a man who said he was heterosexual was a link between a "declared MSM" and a woman, so caught HIV from one and passed it on to the other. Note that neither of these configurations can show who infected who: establishing a direction of infection in phylogenetic analysis is much more difficult than demonstrating linkage. The configuration in which an "undisclosed MSM" was in the middle and was two times more likely than the one where the woman was in the middle.

This study provides a considerably higher estimate of the proportion of ‘heterosexual’ men with HIV in the UK who are in fact "undisclosed MSM" than a previous study in 2014. This estimated the proportion as 6%, though it did estimate that the proportion of "undisclosed MSM" among black African men was as high as 21%.

It would be interesting to see an analysis of all subtypes, especially D and AE. These subtypes are predominant type in east Africa, Middle East and south-east Asia.

The data were presented by Manon Ragonnet-Cronin and colleagues from Edinburgh University at the 2017 Conference on Retroviruses and Opportunistic Infections (CROI) last week.