SYDNEY—The Kirby Institute at Australia’s University of New South Wales has published results of a study showing no transmission of the HIV virus between couples where one partner is HIV positive but undetectable and the other partner is HIV negative.

The new study titled “Opposites Attract” is currently investigating 234 gay male serodiscordant couples: 135 of which are from Australia, 52 from Bangkok, Thailand, and 47 from Rio de Janeiro, Brazil. The study being conducted by Professor Andrew Grulich is in its second year, and the results are extremely encouraging.

“These exciting results directly demonstrating the efficacy of HIV treatment as prevention in gay men are very encouraging,” said Dr. Grulich, program head at the Kirby Institute. “Essentially, what we are seeing among the gay couples enrolled in Opposites Attract is that HIV transmission is quite unlikely when someone’s viral load is undetectable. In fact, no HIV-negative man in the study has contracted HIV from his positive partner.

“It is important to note that although no new HIV infections were seen in early study results, HIV transmission could occur in up to one per cent of gay couples per year having unprotected anal sex. In those couples at highest risk, where the HIV negative partner has unprotected receptive anal sex with ejaculation with his HIV positive partner, the risk could be higher, with a potential HIV transmission rate of up to four per cent per year.”

The results achieved by the “Opposites Attract” match those by the “Partners Study” being conducted by researchers at University College in London. In that study, couples were followed at 75 sites in 14 European countries. Both studies require participants to have condomless sex, and the HIV-negative couples must not be taking medications for pre-exposure or post-exposure prophylaxis. The larger “Partners Study” has documented over 30,000 sex acts with zero transmission of the HIV virus. All of the HIV-positive study participants had been on antiviral therapy for at least five years. About 90 percent had healthy CD4 T-cell counts.

The two studies are the only current research projects to look at seriodiscordant couples in the world. As encouraging as the result are, they are still not conclusive.

Dr. Joel Gallant, associate medical director at the Southwest Care Center in Santa Fe, New Mexico, was quick to remind readers 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 antiviral therapy protects them from HIV but not against other sexually transmitted infections.”

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