HIV drugs called antiretroviral therapy (ART) prevent the virus from making copies of itself. Over time, the amount of virus in the blood can drop so much that standard tests can no longer find it. This is called undetectable viral load.
“If you are undetectable and continue to be undetectable, you cannot sexually transmit HIV to your partners,” Zajesky said. “Not ‘probably not’ or ‘the risk is very low’.” You do not transmit HIV sexually.
That’s the idea behind U=U or Undetectable = Untransmittable, and the science backs it up. A 2023 review in The Lancet examined eight studies involving more than 7,700 couples in which one partner had HIV and the other did not. Researchers found only two possible transmissions when the partner with HIV had fewer than 1,000 copies of the virus per milliliter of blood. In both cases, the partner’s last viral load test was at least 50 days old, so levels may have increased since then.
Zajesky said most of her patients become undetectable four to eight weeks after starting treatment. Still, her team advises patients to use condoms or avoid sex for the first six months of medication to ensure their viral load remains low.
Note that U=U does not protect against other sexually transmitted infections (STIs) or pregnancy. Therefore, other forms of protection such as condoms are still important to ensure the safety of partners.