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There is effectively zero risk of gay men transmitting HIV through sex if the virus is completely suppressed, according to newly published results from the landmark PARTNER2 study.

From 2014 to 2017, the study examined data from 782 gay couples who reported around 77,000 condomless sex acts, and found no HIV transmission from virally suppressed (viral load <200 copies/mL) HIV-positive partners to HIV-negative partners -- meaning the risk of HIV transmission was effectively zero, reported Alison J. Rodger, MD, of University College London in England, and colleagues.

While 288 of 777 HIV-negative men reported condomless sex with other partners, and 15 new HIV infections occurred during the follow-up period -- importantly, none were phylogenetically linked within-couple transmission. This amounted to an HIV transmission rate of precisely zero, albeit with an upper 95% confidence-interval limit of 0.23 per 100 couple-years, the authors wrote.

The study was published Thursday in ; results had also been presented at the International AIDS Conference in Amsterdam.

"Doctors can't say zero, scientists can't say zero," but the chance of transmission through condomless anal sex is "effectively zero," Rodger said at the conference last year.

The results of the study reinforce one of the key tenets of HIV treatment strategies, especially in the U.S. -- namely that "U equals U," or "undetectable equals untransmittable." At the Conference on Retroviruses and Opportunistic Infections (CROI) this March, the National Institute of Allergy and Infectious Diseases (NIAID) director, Anthony Fauci, MD, referenced "U equals U" when discussing details of the plan to end the HIV epidemic by 2030 in the U.S.

Indeed, in an to the Lancet study, Myron Cohen, MD, of the University of North Carolina at Chapel Hill, said that "the pillars of the strategic initiative [to end HIV in the U.S.] include detection and treatment of HIV without delay, a goal clearly justified by the PARTNER2 results."

"During the course of these studies [ and PARTNER2], antiretroviral drugs have become more effective, reliable, durable, easier to take, well tolerated, and much less expensive," Cohen wrote. "The results of the PARTNER2 study provide yet one more catalyst for a universal test-and-treat strategy to provide the full benefits of antiretroviral drugs."

Cohen also touched upon the issue of HIV prevention strategies involving pre-exposure prophylaxis (PrEP), adding that the use of PrEP by the HIV-negative partner was one of the exclusion criteria for eligible couple-years of follow-up in the PARTNER studies.

"[PrEP] has emerged as an important method for HIV prevention and is one of the pillars of the U.S. HIV elimination plan," Cohen wrote.

He cited a survey that showed PrEP was more popular and trusted by men who have sex with men for HIV prevention than "treatment as prevention," and that this reflected "the complex considerations of the HIV-negative partner in a sexual relationship."