Who are trans women acquiring HIV from?

And are trans women's partners a hidden risk population?

There is a lot we don’t know about trans women’s HIV risk, why they are so vulnerable to HIV and who trans women are acquiring HIV from. A study presented at last month’s 9th International AIDS Society Conference on HIV Science (IAS 2017) attempted to answer these questions, but in the process uncovered another mystery. This is whether there is a hidden population of heterosexual men who have sex with trans women and who are themselves at high risk of acquiring and transmitting HIV. One thing the study did make clear, however, is that a lot of HIV infections in trans women may be due to injecting drugs rather than sex.


Trans women are among the people most at risk of HIV infection worldwide. In the United States, it is estimated that 22% of trans women are living with HIV compared with 15% of gay men. And yet we know surprisingly little about how many transgender women there are, how they are acquiring HIV, and who they are getting HIV from. (We know even less about transgender men.)

For instance, only in the last couple of years have researchers realised that transgender women are being drastically under-counted simply because only a minority will answer “yes” to the question “are you a transgender woman?”

Realising this, many researchers now ask two separate questions – what gender were you assigned at birth, and what is your gender now – in order to capture the completeness of the population who are trans or prefer to use other terms such as non-binary.

The study



An umbrella term for people whose gender identity and/or gender expression differs from the sex they were assigned at birth.

cisgender (cis)

A person whose gender identity and expression matches the biological sex they were assigned when they were born. A cisgender person is not transgender.

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.

deoxyribonucleic acid (DNA)

The material in the nucleus of a cell where genetic information is stored.


The complete set of genes or genetic material (information) present in a cell or organism.

Hong-Ha Truong of the University of California, San Francisco conducted a phylogenetic analysis of San Francisco residents diagnosed with HIV between 2000 and 2015. Such analyses are used to uncover linked transmissions by finding individuals whose HIV viruses are closely genetically similar. In this case the study used a “mean pairwise genetic difference” of 0.025% as its criteria for related viruses, meaning that less than one in every 4000 DNA “letters” differed in the genome of the two viruses compared.

The analysis looked at 5200 individuals’ HIV viruses and found that 275 (5.2%) of these viral sequences came from transgender women.

There were 563 phylogenetically-related "clusters" identified and 70 of those (12.4%) included at least one trans woman. (Phylogenetically, “clusters” includes pairs of viruses, but some clusters may also include several tens of people.) A total of 86 trans women were in pairs or clusters. The researchers then categorised the 70 pairs or clusters that included transgender women according to the HIV risk classification of the other person or persons in the cluster.

Up until now, the partners of transgender women have been classed as “men who have sex with men” (MSM), based on the trans women’s birth gender and on the fact that the assumed transmission route in most cases was anal sex. Based on this classification, 54% of pairs/clusters that included a transgender woman also included a man/men who had sex with men; 26% included a person or persons who injected drugs; and 47% included a person or persons who was both a man who had sex with men and who injected drugs. Only one cluster contained a man defined as heterosexual who did not inject drugs. (These percentages add up to more than 100% because clusters may contain several people of different categories.)

Even using this classification, it could be seen that a considerable proportion of the risk of HIV to trans women might be coming, directly or at second-hand, via sharing equipment used to inject drugs.

The researchers then re-classified the risk status of trans women’s partners. They realised that not only do trans women not regard themselves as MSM, but that their partners generally do not either. So they re-classified trans women’s partners as heterosexual men if they were not in a cluster that included other men, unless they injected drugs.

This changed the classification of trans women’s partners dramatically. Forty-seven per cent of clusters containing a trans women then also included a heterosexual man or men who did not inject drugs, and 57% included a heterosexual man or men who did also inject drugs. Only 16% of clusters included MSM who injected drugs, and another 16% MSM who did not.

Dr Truong pointed out that this partner risk profile much more closely resembled that of cisgender women with HIV than MSM. In the 130 clusters containing cisgender women, 58% contained a heterosexual man or men who did not inject drugs, and 41% a man or men who did. Twenty-eight per cent contained MSM and 27% contained MSM who injected drugs.

“The classification of male sexual partners of trans women as MSM may obscure their true sexual networks,” commented Dr Truong, “and also underlines that HIV prevention programmes for MSM may not be applicable to trans women – or their sexual partners.”

This study, then, begs another question – who are these men who have sex with trans women? Are they also at high risk of HIV? And do they also have sex with cis women and represent an HIV risk to them?

Dr Truong told aidsmap.com: “Questions like these prompted our study in the first place. We see high rates of HIV infection in trans women and also a low, but steady, incidence of HIV in cis women, in a city where supposedly three-quarters of HIV infections are in MSM. Are there really scarcely any heterosexual men in San Francisco with HIV? Or have we been misclassifying some of them as MSM?”

She added that their next area of research was to conduct a similar analysis of men who are partners of trans women.


Truong H-H M et al. How are transwomen acquiring HIV? Insights from phylogenetic transmission clusters. 9th International AIDS Society Conference on HIV Science, Paris, Abstract TUPDC0102, July 2017.

View the abstract on the conference website.

Watch the webcast of this session on YouTube.