Public health experts do not condone the practice, but it appears that the risk of transmission from an HIV-positive person to an uninfected sex partner during condomless sex is low if the infected partner has good viral control on antiretroviral therapy.
That’s the tentative conclusion investigators in the PARTNER study have drawn from an interim analysis looking at HIV transmission among serodifferent couples in which the positive partner is taking antiretroviral therapy (ART) and has a plasma viral load below 200 copies/mL, said Dr. Alison Rodger from the Research Department of Infection & Population Health at University College London.
"Overall, we had no linked transmissions during eligible follow-up, giving a transmission rate of zero," she reported at the Conference on Retroviruses and Opportunistic Infections.
Dr. Alison Rodger
However, the investigators cannot say with certainty that the actual risk is zero, as the upper limit of the 95% confidence interval was 0.4 per 100 couple-years, translating into a 10-year risk of 4%, Dr. Rodger said.
Although HIV transmissions did occur during the study, the investigators were able to rule out linked transmissions – that is, transmissions that are phylogenetically confirmed to be the same viral strain.
"The uncertainty is particularly advanced for anal sex, where for the moment we do not have sufficient data to exclude that 1 in 10 [HIV-negative partners] would be infected over a 10-year period," commented coinvestigator Dr. Jens Lundgren from the Centre for Viral Diseases and the Copenhagen HIV Programme at the University of Copenhagen.
The investigators plan to continue the study and conduct new analyses each time an additional 1,000 couple-years have been reached, he said at a briefing following the presentation.
The investigators enrolled serodifferent heterosexual and men having sex with men (MSM) couples who reported having had condomless penetrative anal or vaginal sex in the month prior to study entry, and did not use pre- or postexposure HIV prophylaxis.
Every 6 months during follow-up, each partner fills out a sexual behaviors questionnaire, and the seronegative partner is tested for HIV.
The informed consent for the study included information on the need for consistent condom use and explicit reference to the fact that the HIV-negative partners knew that their partner was positive and that there was risk for HIV transmission.
A total of 767 couples have contributed a total of 894 eligible couple follow-up years (586 in heterosexual couples and 308 in MSM) as of November 2013.
Median ART duration at baseline was 4.8 years, and the median duration of condomless sex reported was 2 years.
Although the risk of transmission across all categories of penetrative anal or vaginal sex was zero, as noted before, the upper limit of the confidence interval for receptive anal sex with ejaculation – the type of sex that carries the highest risk of HIV transmission – extends to 4, which translates into 32% risk of transmission at 10 years, Dr. Rodger said.
"Additional follow-up in MSM is needed through PARTNER phase 2, which will extend through 2017 in gay men only to provide more precise estimates for transmission risk to inform policy, and also to involve individual choice on whether to use condoms or not," Dr. Rodger said.
The PARTNER study is supported by the U.K. National Institute for Health Research. Dr. Rodger and Dr. Lundgren reported having no financial conflicts of interest.