Pre-1996: Even before effective protease-based “cocktail” therapy emerges, early studies show that a higher viral load in someone’s circulating blood correlates with a higher risk of both mother-to-child and sexual transmission.

2008: Based on existing research, the Swiss National AIDS Commission releases “the Swiss Statement,” declaring that a person living with HIV on meds with undetectable virus cannot transmit HIV sexually.

2011: The massive HPTN 052 study—involving more than 1,700 serodiscordant couples from several countries—finds that when the HIV-positive partner’s viral load is consistently suppressed, sexual transmission of the virus to the HIV-negative partner is nearly impossible.

2014: The PARTNER study, looking at more than 58,000 episodes of condomless sex between 900 heterosexual serodiscordant couples in which the HIV-positive partner was on meds with suppressed virus, finds zero instances of HIV transmission. A follow-up study of similar size finds the same to be true among gay male couples.

2016: HIV activists Bruce Richman and Gus Cairns come up with the catchphrase “U=U” to promote the idea that being undetectable means HIV is untransmittable. That same year, Richman starts the Prevention Access Campaign to help spread the news globally. Also, the UK’s Terrence Higgins Trust, the New York City Department of Health, the national AIDS group NASTAD and the federal National Institute of Allergy and Infectious Diseases (NIAID) sign on to U=U.

2017: Activists at the International AIDS Conference in Paris protest, demanding that U=U be recognized and promoted worldwide. At the same conference, leaders from UNAIDS (the Joint United Nations Programme on AIDS) and the U.S. President’s Emergency Plan for AIDS Relief (PEPFAR) as well as NIAID head Anthony Fauci, MD, confirm U=U. Also that year, the Opposites Attract study of 343 serodiscordant gay male couples finds zero instances of HIV transmission amid nearly 17,000 instances of condomless anal sex. And in September, the Centers for Disease Control and Prevention (CDC) declares that people with HIV who are undetectable carry “effectively no risk” of transmitting HIV sexually.

2019: The CDC confirms that viral suppression is “100% effective” for preventing HIV transmission sexually. Fauci calls U=U “the foundation of being able to end the epidemic.” Over the following years, countries around the world begin publicly recognizing U=U and enshrining it in their HIV treatment and prevention guidelines. 

2023: At the International AIDS Society Conference on HIV Science, the World Health Organization (WHO) reaffirmed that people with an undetectable viral load have "zero risk" of transmitting HIV to their sex partners, and those with suppressed but detectable viral load (between 200 and 1,000) have “almost zero or negligible risk."