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(CNN) -- The evidence is in and the message is clear: When someone is HIV positive, taking regular treatment can pretty much zero their chance of spreading the infection to others during sex.
Sexual transmission of HIV is negligible when someone is on treatment, whether they're in a heterosexual or homosexual relationship, according to results from previous studies and now a large-scale study of homosexual men, presented at the ninth International AIDS Conference on HIV Science in Paris on Tuesday.
HIV experts emphasized this aspect of prevention, highlighting the "Undetectable equals Untransmissible" campaign, during a press conference at the international meeting. The campaign works to encourage people worldwide to stay on treatment by ensuring they understand that doing so could mean they cannot infect others.
This message is not aligned with the status quo in terms of the care people infected with HIV receive today, believes Bruce Richman, founder and executive director of Prevention Access Campaign and the "Undetectable = Untransmittable" initiative. "This is transmission-stopping information," he said.
New vaccine results have shown promise at the meeting this week and in recent studies, but are still far from becoming a reality to end the epidemic.
In the largest-ever trial on HIV transmission risk among homosexual men, Australian researchers explored the sex lives and HIV rates of more than 350 homosexual couples where one person is HIV positive. The couples were from Brazil, Thailand and Australia.
Each couple reported their sexual activity when visiting clinics involved in the trial and HIV-negative partners were regularly tested to diagnose any new infections.
The couples participating reported having sex almost 17,000 times without condoms between them over four years, and none of those times resulted in new infections.
"There was not a single linked HIV infection in these couples," said Andrew Grulich, professor of epidemiology at the University of New South Wales in Australia, who led the study. "Nobody became infected from their partner."
Three new infections were discovered during the trial, but analysis of the virus showed they had come from sex outside of the relationships, not from the person on treatment within the couple.
Sex without a condom is not necessarily advised, however, to prevent risk of other sexually transmitted infections (STIs). "This (group) had very high STIs," Grulich told CNN, adding that 20% of the men in the trial developed STIs each year, yet there were zero HIV infections.
Approximately 10% of men had STIs associated with anal sex, which experts had previously thought aided HIV transmission, Gulich said.
The new evidence builds on previous studies on couples where one partner is HIV positive and on treatment, including a landmark study in 2011 that found that treatment can prevent new infections among couples by 96% and a second study in Europe in 2016 showing no transmission at all.
Treatment as prevention is now recommended by the World Health Organization as a key component to include in HIV prevention programs,
The latest trial is the first to explore the benefits of treatment as prevention across multiple continents, showing this approach could be universally applicable. "We wanted to see if this could be applied in different settings where there are also HIV epidemics among homosexual men," said Grulich.
An estimated 68% of new HIV infections in Australia in 2015 were among homosexual men, according to the Australian Federation of AIDS Organizations.
The preventative effects are particularly strong due to the increased risk of transmission that comes with anal sex.
Protective, despite greater risk
"We know transmission risk by anal sex is approximately 10 times higher than risk by vaginal sex," said Grulich, who feared this could lead to some infections during the trial. "This provides reassuring evidence that treatment is as effective in homosexual men," he said.
"This (study) is confirmation of something we have known for some time," said Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Disease, within the US National Institutes of Health.
The fact that transmission risk is clearly greater by anal intercourse shows this is a powerful tool for prevention, he said. "Now is the time to push for it."
International AIDS Society President Linda-Gail Bekker believes working to ensure more people get tested for HIV and therefore treated is a crucial component of the current fight against the virus, but not necessarily the immediate one.
"Let's have a reductionist approach, but I think there is also lag in terms of treatment as prevention. You've got to get things up to scale," she told CNN. In the meantime, she said, the services we already have for people who are uninfected to protect themselves should be pushed.
These currently available services include high-risk groups taking drugs to prevent infection, known as pre-exposure prophylaxis (PrEP), medical male circumcision, which can reduce transmission from women to men by 60%, and condom use. "That will curb the transmission rate more quickly" while we begin treatment as prevention and wait for it to become more widely available globally, she said.
Bekker used the example of Swaziland, where rates of HIV were reported on Monday to have dramatically declined since 2011, according to the US President's Emergency Plan for AIDS Relief. Swaziland has the highest prevalence of HIV in the world, but new infections have almost halved since 2011, after providing treatment and male circumcision for more than 12 million people.
By the time these prevention options are extensively promoted and provided to those who need them, Bekker hopes a vaccine may then finally ready. This would truly eliminate the disease, rather than control it, she believes.
New vaccine promise
Results from a recent vaccine trial, known as the APPROACH trial, were presented at the conference Monday and revealed this type of vaccine could instigate an immune response against HIV when tested on almost 400 volunteers across five countries.
The type used were mosaic vaccines, where components of different HIV viruses are combined together to create an immune response in the body.
Seven different regimens of the vaccine were tested and all elicited an immune response and were well tolerated in the body. One that previously showed promise in animals shined through, giving the strongest response in humans.
The researchers stress, however, that simply because an immune response was created does not mean it will prevent someone becoming infected with HIV.
"The promising, early-stage results from the APPROACH study support further evaluation of these candidate vaccines to assess their ability to protect those at risk of acquiring HIV," said Dr. Dan Barouch, a principal investigator for APPROACH, in a statement.
"A safe and effective HIV vaccine would be a powerful tool to reduce new HIV infections worldwide and help bring about a durable end to the HIV/AIDS pandemic," said Fauci, whose institute supported the trial. "By exploring multiple promising avenues of vaccine development research, we expand our opportunities to achieve these goals."
Bekker added that this is one of three avenues currently being explored to create an effective vaccine against HIV, including one being trialed in her home country, South Africa. Whichever one wins the race, it can't come fast enough.
"Finally, a vaccine will clean (HIV infections) up" after these prevention services are in place, she said. "And hopefully eliminate HIV."