Science Has Spoken: Undetectable Equals Untransmittable

Groundbreaking research supports a progressive idea in HIV treatment and the future of HIV transmission rates.


The consensus is in: Undetectable is Untransmittable (U = U). That is, scientific experts are finally willing to say that the concept of “Undetectable is Untransmittable” for HIV treatment is now “firmly established.” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), writing in JAMA, says an “overwhelming” body of clinical evidence provides a firm basis for accepting the concept as scientifically sound.

In the JAMA commentary, Fauci and colleagues review the results of clinical trials validating U = U. One landmark study, for instance, showed that no linked HIV transmissions occurred among HIV serodifferent heterosexual couples when the partner living with HIV had a durably suppressed viral load. Subsequent studies confirmed the findings and extended them to male-male couples.

The key, the researchers all agree, is to be absolutely adherent to antiretroviral therapy (ART). Viral suppression measured at 6 months after starting therapy is required for U = U. Stopping ART represents a “significant challenge” to successful implementation of U = U. According to the clinical trials, when ART is stopped, viral rebound usually occurs within 2 to 3 weeks. In 2 studies, stopping ART caused viral rebound to levels that would have been associated with increased risk of HIV transmission.

The NIH experts say this consensus has a variety of implications. It gives incentive to people living with HIV to start and adhere to treatment, removes the sense of fear and guilt they may have about harming others, and reduces the risk of legal penalties arising from putting virus-free partners at risk. And because “prevention as control” is a critical tool, the U = U concept can support worldwide efforts to control—or even eliminate—the pandemic.


Eisinger RW, Dieffenbach CW, Fauci AS. JAMA. 2019.

doi: 10.1001/jama.2018.21167. [Epub ahead of print.]

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