Just published findings from the largest study of HIV treatment as prevention among men who have sex with men have added to the existing body of evidence that if people living with HIV take antiretroviral therapy (ART) as prescribed to achieve and maintain an undetectable viral load, there is effectively no risk of sexually transmitting HIV to their HIV-negative partners. The findings from the PARTNER2 study were published online May 2, 2019, in The Lancet.

Prior studies had provided strong evidence of the effect of antiretroviral therapy on the risk of HIV transmission between heterosexual mixed-status (one partner was HIV-positive, the other HIV-negative) couples. However, limited data was available on its effectiveness at preventing HIV transmission among men who have sex with men. These new results mirror those observed in previous studies that antiretroviral treatment that leads to viral suppression prevents sexual transmission of HIV between gay male couples as well as it does among heterosexual couples—an important finding, since anal sex carries a higher risk of transmission than vaginal sex.

The PARTNER2 study built on one of those prior studies and expanded it to involve nearly 800 mixed-status gay couples in 14 countries in Europe between September 2010 and July 2017. The HIV-positive partner in each couple was on treatment and had regular laboratory tests to confirm a viral load of less than 200 copies/mL. The HIV-negative partner was tested regularly for HIV. The couples engaged in sex without condoms and without the HIV-negative partner using HIV pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP). There were no transmissions of HIV within the couples during the study period.

The authors conclude, “Our findings provide conclusive evidence that the risk of HIV transmission through anal sex when HIV viral load is suppressed is effectively zero. Among the 782 serodifferent gay couples followed for almost 1600 eligible couple-years of follow-up, which included more than 76,000 reports of condomless sex, we found zero cases of within-couple HIV transmission. In the absence of ART, on the basis of the frequency and type of sex, for receptive condomless anal sex acts alone approximately 472 transmissions (95% CI 83–714) would have been expected.”

The first of those prior studies demonstrating that a person with a suppressed viral load does not transmit HIV was known as HPTN 052. In 2011 it also found zero linked transmissions of HIV in serodifferent couples when the HIV-positive partner was successfully treated with antiretroviral therapy. Dr. Myron Cohen of the University of North Carolina led that study. In a commentary accompanying the publication of the PARTNER2 results in The Lancet, Dr. Cohen observes, “These important results serve to inspire and challenge us. Timely identification of HIV-infected people and provision of effective treatment lead to near normal health and lifespan and virtual elimination of the risk of HIV transmission.”

“These important findings unequivocally demonstrate that Undetectable=Untransmittable, or U=U — a concept that is integral to the four strategies of Ending the HIV Epidemic: A Plan for America,” observed Dr. Carl Dieffenbach, Director of the Division of AIDS at the NIH’s National Institute of Allergy and Infectious Diseases. “These strategies include getting more people tested and, if they test positive, into HIV care and on treatment. When a person with HIV is in care and HIV treatment durably suppresses their viral load, three important things happen. He or she can live a long and healthy life. He or she won’t transmit the virus. And finally, the stigma that has long been associated with HIV diminishes. All are critical to achieving our goal of reducing the number of new HIV transmissions by 75 percent in five years and 90 percent in 10 years.”

Read or download and share the full open-access article, “Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results of a multicentre, prospective, observational studyExit Disclaimer.”

Read Dr. Myron Cohen’s commentary, “Successful treatment of HIV eliminates sexual transmissionExit Disclaimer.”

Learn more:

• Watch our interview with NIH’s Dr. Carl Dieffenbach after the PARTNER2 findings were first reported last summer at the International AIDS Conference (AIDS 2018).
• Read more about Treatment as Prevention in HIV.gov’s Basics section.
• See NIAID’s “10 Things to Know About HIV Suppression.”