A study of gay male couples where one partner has HIV and is being treated with antiretroviral therapy while the other is HIV-free has shown just how effective virus suppression can be. It demonstrates the obstacle to eliminating AIDS within a lifetime is getting drugs to everyone who needs them.

Professor Alison Rodger of University College London recruited 782 positive/negative couples from 14 Europen countries and had them record how often they had sex, and whether they used condoms, over a period that averaged two years. Immune cell counts and viral load were checked regularly for the HIV-positive partner to confirm the medication's success.

For 97 percent, the blood viral load was undetectable. More than 76,000 acts of condomless anal sex between the partners were reported, along, of course, with many other sexual acts with lower transmission risk. Whether because they were in an open relationship, cheated, or had sex with someone else after a break-up, more than a third of the HIV-negative men also reported at least one act of condomless sex with someone besides their regular partner.

Fifteen of the men (2 percent) who were HIV-negative at the start of the trial contracted the virus during the trial, Rodger reports in The Lancet. However, in every case, virus DNA showed they had different strains from the one their partner had, so it must have been caught elsewhere.

The work demonstrates existing anti-HIV drugs suppress the virus so well they not only provide near-normal lifespans for the people on them, but can virtually eliminate the risk of transmission. “Our findings support the message of the international U=U campaign that an undetectable viral load makes HIV untransmittable,” Rodger told The Guardian.

On the other hand, it is clear there are still many men either unaware of their HIV status, or non-consistent enough in taking virus-suppressant drugs that they can still transmit.

The findings are consistent with previous studies in mostly heterosexual couples where one partner had the virus, and refute the belief there is something uniquely risky about sex between men.

Rodger and co-authors argue the findings emphasize the need to increase testing, not only for the health of those who may have the virus but to prevent new cases. Rodger expressed the hope that the results will reduce HIV stigma, something that is likely to encourage testing.

The biggest problem for HIV control, however, lies in places where getting antiretroviral drugs, or even testing, is difficult. A recent study found HIV transmissions within the European Union peaked in 2004, and have been in subsequent slow decline. However, in Europe as a whole rates have almost doubled since, as a result of a rapid increase in Eastern Europe, primarily through heterosexual transmission. Worldwide, less than 22 million of the estimated 40 million people with HIV are receiving treatment.