HIV medicine, called antiretroviral therapy (ART), stops the virus from making copies of itself. Over time, the amount of virus in the blood can drop so low that standard tests can’t find it. This is called an undetectable viral load.
“When you get undetectable and stay undetectable, you cannot sexually transmit HIV to your partners,” Zajesky said. “Not ‘probably won’t’ or ‘the risk is very small.’ You will not sexually transmit HIV.”
That’s the idea behind U=U, or Undetectable = Untransmittable, and science backs it up. A 2023 review in The Lancet looked at eight studies of more than 7,700 couples where 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, that partner’s last viral load test was at least 50 days old, so their levels may have risen since.
Zajesky said most of her patients become undetectable four to eight weeks after starting treatment. Even so, her team advises patients to use condoms or skip sex for the first six months on medication to make sure their viral load stays low.
Note that U=U does not protect against other sexually transmitted infections (STIs) or pregnancy, so other forms of protection such as condoms are still important to keep partners safe.
