Though sex between partners with different HIV statuses very rarely spreads HIV, experts warn not to throw caution to the wind.

A large-scale study that has documented more than 30,000 sex acts between HIV mixed, or serodiscordant, gay and straight couples has detected zero transmissions of the HIV virus.

All of the HIV-positive partners are on life-saving antiretroviral therapy (ART) and have suppressed viral loads of less than 200 copies per milliliter of blood. Levels are called “undetectable” when they fall below 50 copies per milliliter. All of the HIV-positive study participants had been on ART for at least five years. About 90 percent had healthy CD4 T-cell counts.

Researchers at University College London are overseeing the study, known as Partner. It follows couples at 75 sites in 14 European countries. It requires participants to have condomless sex, and the HIV-negative couples must not be taking medications for pre-exposure or post-exposure prophylaxis.

These findings confirm the results of a famed 2011 study known as HPTN 052. That study, looking almost exclusively at heterosexual couples, showed that taking ART reduced the risk of HIV transmission by 96 percent.

Alison Rodger of University College London presented the preliminary Partner findings last week at the Conference on Retroviruses and Opportunistic Infections in Boston. The big news is that we now know the HPTN findings also apply to gay men and anal sex.

What’s My HIV Transmission Risk? FAQs For Mixed-Status Couples »

The findings have not yet been published in a peer-reviewed journal. And they do not necessarily mean that there is zero risk of contracting HIV from a medicated person with the disease who has a suppressed viral load and healthy T-cell counts.

The study followed more than 700 couples, 39 percent of them gay men. The gay couples reported only 1.5 years of condomless sex, while the heterosexual couples reported having sex without condoms for about twice that long.

During the study, an undisclosed number of HIV-negative partners became positive. But genetic testing revealed that all of those infections came from someone other than their partner. The total number of infections will be disclosed at the end of the study in 2017, Rodger said.

In her presentation, Rodger reported the following conclusions about transmission risk. These are based on current unknowns, which she said further Partner results will help clarify:

  • A receptive anal partner (with ejaculation) may have a risk as high as 32 percent over 10 years
  • A receptive anal partner (without ejaculation) may have a risk as high as 10 percent over 10 years
  • A receptive vaginal partner may have a risk as high as 4 percent over 10 years

The preliminary results of the study offer some insights into the sexual habits of gay and bisexual men. In the U.S., men who have sex with men accounted for two-thirds of the 50,000 new HIV infections in 2011, according to the U.S. Centers for Disease Control and Prevention (CDC).

Learn More: Busting HIV Transmission Myths »

The study showed that:

  • While 3 percent of HIV-negative partners in heterosexual relationships had condomless sex with someone other than their partner, 34 percent of partners in gay relationships did
  • Although about 5.5 percent of heterosexual HIV-negative partners developed a sexually transmitted infection, the number was three times as high for the negative partner in the gay group
  • Although gay men only made up 39 percent of study participants, they represented 53 percent of the sex acts

Dr. Joel Gallant, associate medical director at the Southwest Care Center in Santa Fe, New Mexico, told Healthline that people need to remember that infection can occur even from a partner with an undetectable viral load.

Viral loads are not checked daily, and they do fluctuate, he said. Sometimes, the virus exists in semen even though it cannot be found in blood. So there is no real time means of knowing what is going on inside your partner’s body.

“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,” Gallant said. “They should also remember that ART protects them from HIV but not against other sexually transmitted infections.”

Many factors can influence HIV transmission risks. This study looked at couples who had been together for several years already and transmission had not occurred. Nobody knows exactly what these couples do in their bedrooms or what prevention measures, proven or otherwise, they do or do not take.

Dr. Robert Bolan, medical director of the Los Angeles Gay and Lesbian Center, told Healthline that further studies need to be conducted on more specific sample populations. “You have to caution people that when they are looking at any study … the more you generalize, the more danger you have of being wrong,” he said.

Although medical experts express caution when discussing the findings, HIV-positive activists nationwide say it shines a spotlight on several issues. A debate is raging among national gay commentators about HIV-negative gay men stigmatizing potential partners who are positive.

Read More: How the Media Has Shaped Our Perception of HIV/AIDS »

Many report “serosorting” practices, where one makes assumptions about a potential partner’s status and makes sexual decisions accordingly. But a 2007 study showed that gay men who do this were no more likely to know their own status than those who don’t.

“This is just amazing for us to know—that those of us in treatment and adherent don’t deserve the finger-pointing we sometimes we receive: stigma,” said HIV-positive activist Josh Robbins of Nashville, Tenn. “We are not the responsible party, generally, for new infections—those are from people unknowingly infected and that aren’t being treated regularly or on PrEP.”

Robbins runs a website offering support to people newly diagnosed with HIV.

Pre-exposure prophylaxis, or PrEP, is marketed to negative partners in serodiscordant relationships, but Gallant said he does not recommend it for those in monogamous relationships. “It would not be cost-effective, and the small risk from taking the medication could easily outweigh the benefit. PrEP is a more appropriate intervention for men who are having sex with multiple partners who are seropositive or of unknown status,” he said.

Gallant noted that PrEP gives power to the bottom, or receptive partner. “Condoms are worn by heterosexual men and by gay men engaging in anal sex—’tops’—both of whom are at comparatively low risk of infection.”