No HIV transmissions from HIV-positive partner seen in Australian gay couples study

Andrew Grulich at CROI 2015. Image by The Kirby Institute (
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An Australian-based study of gay male couples of opposite HIV status (serodifferent couples) has so far seen no transmissions from the HIV-positive partner within the couple in a two-year interim analysis.

The Opposites Attract study started recruiting in May 2012. It recruits gay male serodifferent couples regardless of whether the HIV-positive partner is on antiretroviral therapy (ART) or has an undetectable viral load, and also regardless of whether or not they use condoms. It is now recruiting not only in the three Australian cities mentioned (Sydney, Brisbane and Melbourne) but also in Cairns, Australia and (from 2014) in Bangkok, Thailand and Rio de Janeiro, Brazil.

It is currently funded to continue until the end of this year but the researchers are applying for an extension until the end of 2016.



Having sex without condoms, which used to be called ‘unprotected’ or ‘unsafe’ sex. However, it is now recognised that PrEP and U=U are effective HIV prevention tools, without condoms being required. Nonethless, PrEP and U=U do not protect against other STIs. 


A serodifferent couple is one in which one partner has HIV and the other has not. 

detectable viral load

When viral load is detectable, this indicates that HIV is replicating in the body. If the person is taking HIV treatment but their viral load is detectable, the treatment is not working properly. There may still be a risk of HIV transmission to sexual partners.


Insertive anal intercourse refers to the act of penetration during anal intercourse. The insertive partner is the ‘top’. 

By December 2014, by which time the average length spent in the study was one year, 135 couples from Australia had been enrolled, 52 from Bangkok and 47 from Rio. Their median age was 36 (with no difference in the HIV-positive and HIV-negative partners). Many relationships were new, with 39% of couples established for less than a year, 33% for one to five years and 28% for over five years.

At the start of the study, most of the HIV-positive partners were on ART (84%) and virtually all of these had an undetectable viral load (under 200 copies/ml).

Since the study is still enrolling, only the first 152 couples with at least one post-enrolment visit are included in this analysis.

Eighty-eight couples of the 152 couples (58%) reported having condomless anal sex a total of 5905 times over the course of the study so far, an average of 67 times each.

Of the 5905 occasions of sex, the HIV-negative partner reported being the insertive (top) partner 60% of the time and bottom 40% of the time. There were few episodes of condomless anal sex where the HIV-positive partner was known to have a detectable viral load (over 200 copies/ml) – only 237, representing 4% of all episodes of condomless sex.

No transmissions between couples (linked transmissions) have so far been seen in Opposites Attract. (Whether transmissions are linked or not is determined by genetically testing the HIV of any study participant who becomes newly infected to see if it closely resembles their regular partner’s virus.)

Given high rates of non-monogamy (43% defined themselves as being in a non-monogamous relationship) and of sexually transmitted infections diagnosed during the year (11% in the HIV-positive partners and 7% in the HIV-negative ones), it is likely that there have been a number of unlinked infections in the HIV-negative partners, though the researchers have not released this figure.

Because of the relatively small numbers of couples so far enrolled and of condomless sex acts seen, even though there have been no transmissions seen, the researchers have to state the degree of uncertainty of their findings, because no transmission is not necessarily the same as zero chance of transmission. 

Last year, the larger PARTNER study reported no episodes of transmission among 16,400 episodes of anal sex (including condom-protected ones) in gay men. Both Opposites Attract and PARTNER researchers calculated the 95% confidence intervals for the results seen. What this means is that they calculated the odds of zero transmissions being the ‘true’ figure and the maximum number of actual transmissions they might see if they could study every HIV-negative gay man in the country.

The PARTNER scientists had established that there was a 95% chance that, given the size of their study, they might miss up to a 1% annual chance of transmission via anal sex and a 2.5% chance where the negative partner took the passive role. However, these are not the actual chances of transmission: the real chance lies somewhere between zero and these figures, and the true figure may well actually be zero.

Similarly Opposites Attract established that, in this population, given the size of their study, the highest-likely figure for the chance of transmission via condomless anal sex with a positive partner (regardless of viral load) was 4% and where the HIV-negative partner was bottom, 7%. But as with PARTNER, these estimates are likely to get closer to zero as the researchers collect more data.


Grulich A et al. HIV transmission in male serodiscordant couples in Australia, Thailand and Brazil. 2015 Conference on Retroviruses and Opportunistic Infections (CROI), Seattle, USA, abstract 1019LB, 2015.