Substantial proportion of male HIV infections labelled as 'heterosexual exposure' in UK probably due to sex with other men

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Up to a fifth of HIV infections among black African men initially classified as 'heterosexual exposure' in the UK are likely to have been acquired as a result of sex with other men, investigators report in the online edition of AIDS. Using a technique called phylogenetic analysis, the authors identified clusters of HIV transmissions involving people diagnosed with HIV in the UK between 1996 and 2008. Overall, 29% of heterosexual people were in transmission clusters that only involved men who have sex with men (MSM). The authors estimate that overall 6% of HIV infections involving heterosexual men are misclassified and are actually the result of sex with other men. But the proportion of misclassified infections involving black African heterosexual men could be as high as 21%.

The authors suggest that some men are not disclosing that they have sex with other men when testing for HIV.

“This study is the first to attempt to quantify nondisclosure of homosexually acquired infections reported through national surveillance and illustrates how phylogenetic analysis can complement traditional epidemiological analysis,” explain the investigators.



In HIV, different strains which can be grouped according to their genes. HIV-1 is classified into three ‘groups,’ M, N, and O. Most HIV-1 is in group M which is further divided into subtypes, A, B, C and D etc. Subtype B is most common in Europe and North America, whilst A, C and D are most important worldwide.


A variant characterised by a specific genotype.


transmission cluster

By comparing the genetic sequence of the virus in different individuals, scientists can identify viruses that are closely related. A transmission cluster is a group of people who have similar strains of the virus, which suggests (but does not prove) HIV transmission between those individuals.

phylogenetic analysis

The comparison of the genetic sequence of the virus in different individuals in order to determine the likelihood that two or more samples are related. This involves creating a hypothetical diagram (known as a phylogenetic tree) that estimates how closely related the samples of HIV taken from different individuals are. Phylogenetic analysis is not a reliable way to prove that one individual has infected another, but may identify transmission clusters, which can be useful for public health interventions.


The study of the causes of a disease, its distribution within a population, and measures for control and prevention. Epidemiology focuses on groups rather than individuals.

HIV prevalence in the UK is higher in MSM than any other group. Most HIV infections in MSM involve HIV subtype-B strains. However, between 10 and 13% of recent infections in heterosexual people also involve subtype-B strains. The majority of these (55%) are in men.

A team of UK investigators hypothesised that at least some HIV subtype-B strain heterosexual infections in men have been misclassified and were actually acquired through sex with other men.

They therefore studied the genetic structure of the virus acquired by 22,500 people newly diagnosed with HIV in the UK between 1996 and 2008. Viral sequences were linked with anonymous demographic information obtained from the UK Collaborative HIV Cohort Study (UK CHIC) and the HIV and AIDS Reporting System at Public Health England (HARS). This allowed the authors to identify transmission clusters.

A total of 61% of infections were genetically linked to at least one other infection and there were 2860 transmission clusters.

Overall, 56% of heterosexual people could be placed within a transmission cluster.

Just over half (52%) of these heterosexuals belonged to transmission networks that only involved other heterosexuals.

However, 31% of linked heterosexual infections belonged to clusters that involved both heterosexuals and MSM. Moreover, 29% were solitary heterosexuals in transmission networks that were otherwise exclusively MSM.

Heterosexuals linked to exclusively MSM transmission clusters were more likely to be men than women (p < 0.001). The investigators estimated that 6% of all HIV infections among heterosexual men with subtype-B infection are due to homosexual contact.

Heterosexual men of black African origin were the racial group most likely to be linked to exclusively MSM transmission networks (21% vs 10% white men).

“Such observations support the notion that black African men are less likely to disclose sex between men as a route of potential exposure compared to other ethnic groups,” note the authors. “Factors hampering disclosure of same-sex sexuality include social-cultural barriers and experiences of discrimination. This seems to be particularly true for MSM of black or minority ethnic groups.”


Hue S et al. Phylogenetic analyses reveal HIV-1 infections between men misclassified as heterosexual transmissions. AIDS 28, online edition. DOI. 10: 1097/QAD.0000000000000383, 2014.