Researchers at a world AIDS conference Monday stood up to cheer a trial proving that early use of drugs to treat HIV all but eliminates the risk of transmitting the killer virus through heterosexual intercourse.
Experts said the bold experiment marked a turning point in a disease that has claimed some 30 million lives and left 34 million people infected with HIV.
They said widening access to antiretroviral drugs would deliver a double whammy: millions of people infected by the human immunodeficiency virus (HIV) would be saved and millions of new infections would be averted.
But they stressed that the message of safe sex that has shadowed AIDS for the past three decades should not be abandoned.
And before "treatment as prevention" is launched as a strategy, it would need carefully crafted guidelines and billions of dollars in support, at a time when funding for AIDS is faltering.
"We are at an important tipping point," Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases (NIAID), told AFP.
"There is an extraordinary window of opportunity and the sooner we act on it and the more robustly we act on it, the quicker we'll achieve the goal of turning around the epidemic," he said.
"Now's the time to translate this science into action that can break the back of the epidemic," the group Medecins Sans Frontieres (Doctors Without Borders) said. "The clock is ticking."
Casting aside their traditional reserve, delegates cheered and gave a standing ovation to Myron Cohen, a professor at the University of North Carolina at Chapel Hill, who led the landmark HPTN 052 trial.
The data had been initially unveiled to the press in May on the orders of monitors, who said the results were so startling that all participants had to be advised of the outcome on ethical grounds.
Its presentation at the world's top scientific conference on AIDS, and concurrent publication in the peer-reviewed New England Journal of Medicine, now enshrines it as a benchmark to which policymakers will refer.
HPTN 052 was carried out among 1,763 couples in nine countries where one partner was infected with HIV while the other was HIV-free.
When the infected partner was given an early start on HIV drugs, this slashed the risk of transmitting the virus to the other by 96 percent -- a figure readily comparable to the effectiveness of a condom.
Alongside the HPTN presentation were two other trials, made public last week, which found a reduction of up to 73 percent in risk when the non-infected partner, as opposed to the infected partner, took the antiretrovirals.
Experts said many tasks lay ahead in transposing a trial, which takes place in carefully controlled conditions where couples are counselled and supported, into the messy reality of everyday life.
The UN's World Health Organisation (WHO) on Monday said it would delay plans to revise its guidelines on antiretrovirals to give it time to ponder the radically altered landscape.
However, the use-a-condom message will remain in place, said the WHO's Gottfried Hirnschall.
"Messaging is critical and clearly we not just talking about a single intervention, we are talking about a combination prevention," he told a press conference.
"We would never at this point want to say, let's do just one thing and say 'don't use condoms'."
Another question is funding. Around 16 billion dollars have been earmarked to fight AIDS this year, compared to needs estimated at 22 to 24 billion dollars.
"The priority right now is reaching people who really need treatment," argued Michel Kazatchkine, head of the Global Fund to Fight AIDS, Tuberculosis and Malaria.
"(...) Putting everyone on treatment is not a prospect, either from the resource point of view or from the operational point of view. Half of the people who are infected aren't even aware of their (infection) status."
The four-day conference, staged once every two years by the International AIDS Society (IAS), gathers 5,500 specialists, ranging from virologists to pharmacologists and disease trackers. It ends on Wednesday in Rome.