The Europe-wide study monitored almost 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.
Among the participating couples, antiviral therapy was found to be so protective that it was estimated to have prevented about 470 HIV infections, even though condoms were never used.
"Whether men who are in monogamous relationships in these circumstances chose to use or not to use condoms is up to them, but there is no need to do so to prevent HIV transmission to the negative partner", said the new study's lead author, Dr. Alison Rodger. The "undetectable=untransmittable" concept, shorthanded as "U=U", is relatively young campaign launched in 2016 to try to help explain that medical suppression of HIV so that its viral levels don't show up in blood tests means that it can not be transmitted to other people.
"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", she continued.
Since the start of the AIDS epidemic in the 1980s, more than 77 million people have become infected with HIV worldwide. There too, there were no transmissions of HIV from an infected partner if that partner was on drug therapy that effectively suppressed the virus.
Individuals now on ART must take medication nearly every day for the rest of their lives, and treatment is often disrupted for a variety of reasons.
The researchers concluded that, with the drugs, the HIV virus, which leads to AIDS, could not be passed on through unprotected sex.
The CCR5 gene provides a pathway for the HIV virus to harm a person's immune system, and become infected with the virus.
"Yet maximising the benefits of ART has proven daunting: fear, stigma, homophobia, and other adverse social forces continue to compromise HIV treatment". "Our results give equivalence of evidence for gay men as for heterosexual couples and indicate that the risk of HIV transmission when HIV viral load is suppressed is effectively zero for both anal and vaginal sex". It also found zero risk.
A key point of the study was the fact that it did find HIV infections in 15 men among the 972 gay couples, but genetic analysis showed that their infections were with strains of HIV from another sexual partner. Of those cases, more than 35 million died from AIDS-related illnesses.
While the number of people killed by the virus is declining annually, and the number of people receiving antiretroviral treatment is one the rise, global health experts say the number of new infections per year is still stubbornly high at 1.8 million.