What is the risk of spreading HIV if I am undetectable?

The ongoing PARTNER study, which is seeking a definitive answer to this question, has so far found zero transmissions from an HIV-positive person who has an undetectable viral load (one who is receiving antiretroviral therapy and has a viral load under 200 copies per milliliter of blood) to his or her HIV-negative partner, which is great news. The study to date has covered more than 40,000 sex acts of over 700 mixed (or serodiscordant) couples, of which about 40 percent are same-sex. These couples, spread across 75 locations in Europe, have engaged in vaginal and/or anal sex that was at least occasionally condomless. The negative partners included in the analysis did not use prevention strategies involving HIV drugs, such as pre- or post-exposure prophylaxis, during this time.

However, zero transmissions does not mean a zero percent chance of transmission. At a conference in early 2014, study researchers reported a 95 percent confidence rate in these results due to the limited size and length of time of the study. Because of this uncertainty, researchers still estimate that one in 10 negative partners have a maximum 5 percent risk of contracting HIV over the course of 10 years. This puts the risk of HIV infection from vaginal sex with ejaculation at 2 percent per year; receptive anal sex at 2.5 percent per year; and receptive anal sex with ejaculation at 4 percent per year. But these are worst-case scenarios. Researchers may find these rates to be much lower as more data becomes available.

The researchers also stressed that because of the relatively small number of anal sex acts reported, it is not yet possible to be certain about the risks for HIV-negative men who engage in receptive anal sex with undetectable HIV-positive men. PARTNER continues to recruit gay couples and will release more definitive results in 2017.

In conclusion, the risk of contracting HIV from an undetectable partner is extremely low. Wearing a condom will lower this risk even further and will protect against other sexually transmitted infections. But it’s not 100 percent certain that there is no risk.