When someone has HIV, the amount of virus in their blood is measured using something called viral load. Modern HIV treatment is so powerful that it can reduce this to a level so low it wouldn’t be detected on a HIV test. This is why we say “undetectable”.
When someone is undetectable, their HIV is untransmittable. This is also known as “U=U”. What does that mean? The science has shown that being undetectable completely stops HIV transmission through sex. It doesn’t reduce risk – it eliminates it. U=U is 100% effective at preventing HIV in these cases.*
This data comes from large-scale clinical trials conducted over multiple years in countries across the world. These studies used thousands of people in mixed status relationships (i.e. one HIV-positive and one HIV-negative partner) who were having condomless sex.
If your partner is undetectable, then you don’t need PrEP or condoms to prevent HIV with them. However, condoms still protect against STIs. PrEP can also protect you against HIV if you have other partners.
* This 100% figure is for sexual transmission only. We know that being undetectable also reduces HIV risk when sharing equipment to use drugs, but the exact amount isn’t clear.