Undetectable = Untransmittable (U=U)
Undetectable = Untransmittable, usually written U=U, means that a person living with HIV who takes treatment and maintains an undetectable viral load does not transmit HIV to sexual partners. It is not a hopeful slogan; it is a conclusion backed by large, rigorous studies.
What "undetectable" means
HIV treatment (antiretroviral therapy) reduces the amount of virus in the blood. When it drops below the level a standard test can measure, the person is said to have an undetectable viral load. Reaching this state usually takes up to six months of consistent treatment, and it must be maintained through ongoing adherence.
The evidence behind U=U
Major studies followed thousands of couples where one partner had HIV and was virally suppressed and the other did not. Across many thousands of acts of sex without condoms, there were zero linked HIV transmissions from a partner who was durably undetectable. On the strength of this evidence, the CDC and health bodies worldwide endorse U=U.
What U=U does and does not cover
- It applies to sexual transmission.
- It depends on the viral load being durably undetectable, which means sustained treatment and adherence, not a single good test result.
- It does not protect against other sexually transmitted infections.
- The evidence is strongest for sexual transmission; for shared needles, treatment still lowers risk but U=U is a claim specifically about sex.
Why it matters
U=U transforms both prevention and stigma. It means effective treatment is also prevention, that mixed-status couples can have the relationships they want, and that an HIV diagnosis, with treatment, does not mean a person is a risk to their partners. In this calculator, indicating that a positive partner is undetectable reduces the estimated risk to effectively zero, reflecting this science.