HIV most often passed on by younger gay men who have undiagnosed HIV, are not on treatment, and have an ongoing partnership

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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).


mathematical models

A range of complex mathematical techniques which aim to simulate a sequence of likely future events, in order to estimate the impact of a health intervention or the spread of an infection.

primary infection

In HIV, usually defined as the first six months of infection.


Having no symptoms.


Choosing sexual partners of the same HIV status, or restricting condomless sex to partners of the same HIV status. As a risk reduction strategy, the drawback for HIV-negative people is that they can only be certain of their HIV status when they last took a test, whereas HIV-positive people can be confident they know their status

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.

Men with undiagnosed HIV

Men who are living with HIV without being aware of it cannot take HIV treatment and cannot make informed decisions about their sexual behaviour, so are more likely to pass HIV on. Around one in five gay men living with HIV in the UK – a little under 8000 men – are undiagnosed.

The modelling found that undiagnosed men made a disproportionate contribution to onward transmission, with around 63% (credible range: 49 to 80%) of transmissions coming from them. Moreover, most of the undiagnosed men passing on HIV have a CD4 count above 350 cells/mm3 and so are unlikely to notice problems with their health.

Within the group of men living with HIV who are not taking HIV treatment, a majority are undiagnosed. In this analysis, around 85% of HIV transmissions come from men who are not taking HIV treatment (credible range: 78 to 92%). 

Younger men

The researchers divided adult gay men into two age bands: those aged 15 to 34, and those aged 35 to 64. Although the majority of men living with HIV are in the older age band, younger men living with HIV are less likely to be diagnosed and may have more sex.

According to this modelling, around 62% of HIV transmissions originate in men aged 15 to 34 (credible range: 47 to 74%).

Men with more sexual partners

Within each age band, the researchers created two groups – those with a relatively high level of sexual activity (including at least two new sexual partners each year) and those with less sexual activity. While only around 40% of all gay men are in the high-activity group, HIV is more prevalent within this group and these men have more sexual partners to potentially pass it on to.

Around 80% of transmissions were attributed to men in the high-activity group (credible range: 70 to 87%).

Men in repeat sexual partnerships

The model distinguished between one-off sexual partnerships and repeat sexual partnerships (multiple sex acts with the same partner). Repeat sexual partnerships include both ongoing romantic relationships and ‘fuck buddies’. Also, some men in repeat sexual partnerships have one-off partnerships at the same time.

Men in repeat sexual partnerships may be at greater risk of being involved in HIV transmission because they are less likely to use condoms – even without accurate knowledge of each partner’s HIV status – and because a repeat sexual partnership involves more acts of sexual intercourse.

The modelling found that the vast majority of HIV infections occurred in repeat sexual partnerships – around 90% (credible range: 87 to 93%).

But most of these (around 69%) originated in men who had a relatively high level of sexual activity, in other words at least two new partners a year. The circumstances of men in this group could include men in ongoing relationships who also have casual partners, men having a succession of short-lived relationships and men who have several ‘fuck buddies’. 

Men with primary HIV infection

Primary infection refers to the first few months of infection, when individuals have exceptionally high viral loads. The transmission risk during a single sexual act is much greater than at later stages (the researchers assumed it was nine times greater). However, the duration of primary infection is relatively short (around three months) and as a result there are relatively few men who have primary HIV infection at any one point in time (around 500). For these reasons there is debate about the impact that primary infection has on the HIV epidemic.

In this model, HIV transmission that originates in men who themselves have primary infection was estimated to make up around 10% of all HIV transmissions (credible range: 3 to 28%). This is much lower than the estimates produced by some other modelling exercises – including one which found that 48% of infections came from men with primary infection.


“The majority of new HIV infections among MSM in the United Kingdom during 2001-2020 is expected to be accounted for by a small group of highly sexually active individuals under the age of 35 years, living with undiagnosed HIV in the asymptomatic stage,” conclude the authors.

They urge an intensification of interventions which aim to increase the uptake of HIV testing in this group. Those testing HIV positive should be efficiently linked to medical care and offered HIV treatment.

Moreover, they draw attention to the large number of transmissions occurring within some sort of ongoing relationship. “It may be sensible for policy makers to consider interventions that aim specifically at reducing transmission within long-term, steady relationships such as encouraging steady partners to test together at the beginning of their relationship or to test immediately after breaking up,” they say.


Punyacharoensin N et al. Modelling the HIV epidemic among MSM in the United Kingdom: quantifying the contributions to HIV transmission to better inform prevention initiatives. AIDS, 2014. doi: 10.1097/QAD.0000000000000525