untransmittable

Untransmittable is a public health term used to describe the situation in which a person living with HIV who is on effective antiretroviral therapy (ART) and has achieved an undetectable viral load does not transmit the virus to sexual partners. The phrase is commonly summarized as “undetectable equals untransmittable” (U=U). It is used to reduce stigma and to inform people about the impact of sustained viral suppression.

Undetectable typically means that HIV RNA in the blood is below the limit of detection of standard laboratory tests, often 50 copies/mL or fewer. When viral load remains suppressed, the probability of sexual transmission is effectively negligible, as shown in several large studies involving serodiscordant couples (where one partner is HIV-positive and the other is HIV-negative) across various routes of sex.

Key evidence comes from studies such as PARTNER and PARTNER2, which followed thousands of condomless sex acts and found no linked HIV transmissions when the HIV-positive partner maintained an undetectable viral load. Similar findings have been reported in other large studies, including Opposites Attract. These results have informed clinical guidelines and public health messaging.

The U=U concept originated with the Prevention Access Campaign in 2012 and has been endorsed by major health organizations, including UNAIDS, the World Health Organization, and national health agencies. It supports the idea that effective ART and viral suppression can dramatically reduce sexual transmission risk and improve quality of life for people living with HIV.

Limitations include the need for sustained adherence, the possibility of viral rebound, and the fact that U=U applies to sexual transmission and not to other routes (for example, needle sharing or perinatal transmission). It also does not eliminate the risk of other sexually transmitted infections.