The risk is especially low for straight couples, according to the study.
For the study, published in the Journal of the American Medical Association (JAMA), the researchers followed 900 straight and gay couples.
The couples were serodifferent — meaning thereby that one partner was HIV-positive while the other was HIV-negative — and the HIV-positive partner was using suppressive antiretroviral therapy and also reported condomless sex.
During a median follow up of 1.3 years per couple, there were no documented cases of within-couple HIV transmission, the findings showed.
The researchers, however, noted that appreciable levels of HIV transmission risk cannot be excluded, particularly for anal sex and when considered from the perspective of a cumulative risk over several years.
“Although these results cannot directly provide an answer to the question of whether it is safe for serodifferent couples to practice condomless sex, this study provides informative data (especially for heterosexuals) for couples to base their personal acceptability of risk on,” the researchers said.
The study was conducted at 75 clinical sites in 14 European countries between September 2010 to May 2014.
Alison Rodger from University College London and colleagues evaluated the rate of within-couple HIV transmission during periods of sex without condoms and when the HIV-positive partner had HIV-1 RNA load less than 200 copies/ml.
At study entry, couples reported condomless sex for a median of two years.
Condomless sex with other partners was reported by 108 HIV-negative men who have sex with men (33 per cent) and 21 heterosexuals (four per cent).
During follow up, couples reported condomless sex a median of 37 times per year, with men who have sex with men (MSM) couples reporting approximately 22,000 condomless sex acts and heterosexuals approximately 36,000.
Although 11 HIV-negative partners became HIV-positive (10 gay; one straight) eight of them reported condomless sex with other partners.
No molecular characteristics that indicate whether a virus is similar or different from another linked transmissions occurred over eligible couple-years of follow-up, giving a rate of within-couple HIV transmission of zero, the study said.