HIV-suppressing medication can make the AIDS virus "untransmittable" even among couples who have sex without using condoms, new research showed Friday.
The Europe-wide study monitored nearly 1,000 gay male couples over a period of eight years, where one partner was HIV-positive and receiving antiretroviral (ART) treatment, while the other was HIV negative.
Doctors did not find a single case of in-couple HIV transmission within that time, raising hopes that widespread ART programmes could eventually end new infections.
"Our findings provide conclusive evidence for gay men that the risk of HIV transmission with suppressive ART is zero," said Alison Rodger, from University College London, who co-led the research published in The Lancet.
"They support the message... that an undetectable viral load makes HIV untransmittable. This powerful message can help end the HIV pandemic by preventing HIV transmission, and tackling the stigma and discrimination that many people with HIV face."
Researchers estimate that ART prevented around 470 HIV transmissions within couples during the study period.
HIV and the fatal illnesses it provokes remain one of the world's largest health crises despite much progress in recent years.
More than 21 million people currently receive regular ART medication, which suppresses the virus -- only around 59 percent of global HIV sufferers.
The authors of the study noted several limitations, including that the average age of the HIV-negative men was 38. Most HIV transmissions occur in people aged under 25.
Individuals currently on ART must take medication almost every day for the rest of their lives, and treatment is often disrupted for a variety of reasons.
But the fact that couples can have unprotected sex for years without passing on the virus was still worth noting, experts said.
'There is no risk'
"Timely identification of HIV-infected people and provision of effective treatment leads to near normal health and virtual elimination of the risk of HIV transmission," said Myron Cohen, from the UNC Institute of Global Health and Infectious Diseases.
"Yet maximising the benefits of ART has proven daunting: fear, stigma, homophobia, and other adverse social forces continue to compromise HIV treatment."
The study shows "that we can pass the message 'there is no risk'," said Aurelien Beaucamp, the head of the French lobby group Aides.
The UN goal is for 90 percent of HIV-positive people to know their status by 2020. Of these, at least 90 percent must receive ART, and the HIV virus be suppressed in 90 percent of those.
AIDS has killed 35 million people since it emerged in the 1980s and 78 million people have been infected with HIV.
There were 1.8 million new HIV infections, down from 1.9 million in 2016 and 3.4 million at the peak of the epidemic in 1996, according to UNAIDS.
The number of deaths dropped by 50,000 year-on-year to 940,000, compared to 1.9 million in 2005 when a mere 2.1 million infected people had access to life-lengthening ART.
But for this, money is needed. And the global effort is short about $7 billion (six billion euros) per year, according to UNAIDS.