From NIH.gov…
Extensive evidence from HIV prevention research studies has firmly established that “Undetectable Equals Untransmittable,” or U=U. This means that people living with HIV who achieve and maintain an undetectable viral load — the amount of virus in their blood — by taking antiretroviral therapy (ART) as prescribed do not sexually transmit HIV to others. The U.S. Centers for Disease Control and Prevention estimates this strategy is 100% effective against the sexual transmission of HIV.
“U=U has been validated repeatedly by numerous studies as a safe and effective means of preventing the sexual transmission of HIV,” said Anthony S. Fauci, M.D., NIAID Director. “The increased understanding and acceptance of U=U is encouraging because HIV treatment as prevention is a foundation of efforts to end the epidemic in the United States and around the world. This public health message has the power to reduce stigma, protect the health of people living with HIV and prevent sexual transmission of HIV to others.”