For couples where one partner carries human immunodeficiency virus (HIV), determining a comfortable level of sexual activity and protection can be difficult, as there is always the risk that the HIV-negative partner will contract the deadly disease. Now, preliminary results from a new study show that this risk may be lower than expected, though researchers are far from saying it’s safe.

Called the Partner study, it follows both gay and straight couples in which one partner is HIV-positive, known serodiscordant, and tracks the kind of sex they have and whether the HIV-negative partner contracts the disease. Preliminary evidence of more than 700 couples recruited within the last five months shows that none of the HIV-negative partners contracted the disease after over 30,000 sex acts were recorded. Thirty-nine percent of the participants were gay men, and reported practicing condomless sex for an average of one-and-a-half years, while the straight couples went without condoms for almost twice as long, Healthline reported.  

The researchers discovered that some of the partners had become positive with the infection, however, genetic testing showed that the virus didn’t come from their partners. Still, they determined that receptive anal partners risked HIV transmission by as much as 32 percent when their partner ejaculated, and 10 percent without ejaculation over a 10-year course. Meanwhile, receptive vaginal partners risked transmission by as much as four percent over 10 years. Official rates of transmission, and risk, will be announced at the end of the study, in 2017.

According to the National Institutes of Health (NIH), someone with HIV can reduce their risk of transmitting the virus to a partner by undergoing antiretroviral therapy (ART), which is also one of the standard forms of treatment. Taking these medications — a cocktail of meds tailored to the viral strain — can lower a person’s viral load. Indeed, all of the participants in the study were on ART, and more than 85 percent of participants in all groups had undetectable levels (considered 50 copies per milliliter or below) of the virus — all of them had less than 200 copies per milliliter. Over 84 percent of them also had healthy CD4 T-cell counts.

But even with ART and an undetectable viral load, the risk of transmission persists. The NIH suggests that partners of HIV patients not only wear a condom but also try pre-exposure prophylaxis, a preventative pill treatment that uses two HIV medications. But as always speak to a doctor first. Dr. Joel Gallant, associate medical director at the Southwest Care Center in Santa Fe, New Mexico, told Healthline that the therapy might not be cost-effective for couples in monogamous relationships. He suggests using condoms no matter what, as undetectable viral loads may show in the blood but not in the semen.

“Seronegative men should still consider using condoms with a partner who has an undetectable viral load, especially for the highest risk activity: receptive anal sex with ejaculation,” he told Healthline. “They should also remember that ART protects them from HIV but not against other sexually transmitted infections.”