High HIV incidence from non-primary partners and low PEP and PrEP use seen in PARTNER study

HIV incidence among the HIV-negative gay men in the PARTNER 1 and 2 studies, due to sex with partners outside the main relationship, was high, and very high in partners who admitted having condomless anal sex with non-primary partners, the 16th European AIDS Conference (EACS 2017) heard recently.

The conference heard that even now, in the latest data from the PARTNER 2 study, only 5% of HIV-negative participants are taking pre-exposure prophylaxis (PrEP), even though over a third have had condomless anal sex with non-primary partners.

The data were presented by Valentina Cambiano of University College London, who is one of the investigators in the PARTNER studies.



Having sex without condoms, which used to be called ‘unprotected’ or ‘unsafe’ sex. However, it is now recognised that PrEP and U=U are effective HIV prevention tools, without condoms being required. Nonethless, PrEP and U=U do not protect against other STIs. 

post-exposure prophylaxis (PEP)

A month-long course of antiretroviral medicines taken after exposure or possible exposure to HIV, to reduce the risk of acquiring HIV.

Undetectable = Untransmittable (U=U)

U=U stands for Undetectable = Untransmittable. It means that when a person living with HIV is on regular treatment that lowers the amount of virus in their body to undetectable levels, there is zero risk of passing on HIV to their partners. The low level of virus is described as an undetectable viral load. 

virological suppression

Halting of the function or replication of a virus. In HIV, optimal viral suppression is measured as the reduction of viral load (HIV RNA) to undetectable levels and is the goal of antiretroviral therapy.

treatment as prevention (TasP)

A public health strategy involving the prompt provision of antiretroviral treatment in people with HIV in order to reduce their risk of transmitting the virus to others through sex.

PARTNER is arguably one of the most significant HIV prevention studies ever conducted. It studies couples where one partner has HIV and the other does not and its primary aim is to try to quantify the risk of transmission from a person on HIV treatment who has a fully suppressed viral load. While enrolment of heterosexual couples stopped at the end of PARTNER 1 in 2014, it was decided more data on gay men was needed so PARTNER 2, for gay couples only, is still underway.

PARTNER made headlines when in 2014, and again in 2016, the researchers confirmed that there had been no transmissions from an HIV-positive partner who was on antiretroviral therapy and virally suppressed in, by 2016, an estimated 58,213 condomless sex acts. These data allowed the researchers to establish the maximum possible likelihood of transmission, and to announce that, most likely, the chance of an HIV-positive partner with a fully suppressed viral load of below 200 copies/ml passing on HIV was zero, or statistically indistinguishable from it.

PARTNER, and other studies like Opposites Attract and HPTN 052, have provided the evidence base for the success of ‘Treatment as prevention’ and for the U=U (Undetectable = Untransmittable) campaign.

However, there were HIV infections in PARTNER: eleven of them by 2016, ten in gay men. In all cases, however, phylogenetic testing showed that the infecting virus came from someone other than the primary partner. Eight of the eleven infected people told researchers they had had condomless sex with partners other than their primary partner: the other three must have done too.

Cambiano told the conference that multiplying the number of infections by the amount of time people were in follow-up before infection leads to an estimated HIV incidence of 2.3% a year – quite high. However, if only the eight men who admitted to having condomless anal sex with other partners are counted, then incidence becomes very high – 7.2% a year, or higher than that seen in the placebo arm of the IPERGAY PrEP trial.

In short, just because your main partner is undetectable, it does not mean you are safe from HIV if you have condomless sex elsewhere. In these cases, it would make sense to use post-exposure prophylaxis (PEP) or PrEP – but how many in PARTNER were doing so?

Cambiano and colleagues studied PEP and PrEP use in the 737 HIV-negative partners who had answered both the baseline sexual behaviour questionnaire and a questionnaire on at least one follow-up visit.

They had been in the study on average 1.6 years and had averaged one year of condomless anal sex with their primary HIV-positive partner. Thirty-five per cent said they had had condomless anal sex with other partners and 22% had been diagnosed with a sexually transmitted infection other than HIV. The researchers estimated that on average they had 35 acts of condomless anal sex per year with other partners. They also worked out that the total number of condomless anal sex acts with main partners now amounted to 69,098 acts, strengthening the conclusion that HIV transmission is not taking place when the main partner has undetectable viral load.

At baseline, in both phases of the study, over one in six men (17.5% in PARTNER 1 and 17.9% in PARTNER 2) had ever taken PEP. But far fewer had ever taken PrEP; only 1.5% in PARTNER 1 and 3.9% in PARTNER 2. This is not that surprising as PARTNER is a mainly European study, and PARTNER 1 recruited in 2010 and PARTNER 2 in 2014.

During the studies a further 3% in PARTNER 1 and 4.6% in Partner 2 used PEP, and 3% and 5% used PrEP. This means that in PARTNER 2 8.2% of the HIV-negative partners used PEP and/or PrEP.

However, if the partners admitted to having had condomless anal sex with other men in the follow-up questionnaire, then they were twice as likely to have used PEP and three times as likely to take PrEP: 8.7% of these men took PEP and 10.3% were using PrEP, meaning that nearly 16% had used PEP and/or PrEP.

The 10.3% using PrEP numbered 253 men. A majority (58%) were in the UK, with smaller numbers in other countries: 11% each in France and Switzerland, 8% each in Germany and the Netherlands, and 4% in Spain.

This finding meant that in men reporting condomless anal sex with outside men, PEP or PrEP only covered 12% of sex acts that risked HIV infection and only 1.5% of acts in men who did not report it. It can easily be seen that if the negative partners had covered, say, 80% of their possibly risky sex acts with PEP or PrEP instead of 16%, then HIV incidence would have been considerably lower.

Valentina Cambiano commented that the low level of PEP and PrEP use and the high HIV incidence seen from sex outside the main relationship were of concern.

“PrEP eligibility discussions with HIV-negative MSM [men who have sex with men] should ensure that risks from all sexual contacts are taken into consideration, and routes to securing PrEP discussed,” she added.


Cambiano V et al. Use of PEP and PrEP among HIV Negative MSM in the PARTNER Study. 16th European AIDS Conference, 25-27 October, Milan, abstract PS11/4, 2017.