If your partner has HIV, takes treatment every day and has an undetectable viral load, sex with them carries no risk of HIV. That is one of the most important findings in HIV prevention, and it is backed by large studies and by the CDC and the World Health Organization.

What “undetectable” means

Viral load is the amount of HIV in the blood. HIV medicine, called antiretroviral therapy or ART, stops the virus from copying itself, so the viral load falls. When it is so low that a standard test can’t measure it, it is called undetectable. For U=U, the line is under 200 copies per milliliter of blood.

Undetectable does not mean the virus is gone. It means treatment is keeping it under control. If someone stops taking their medicine, the viral load can rise again, and so can the chance of passing HIV on.

The evidence

Researchers followed couples where one partner had HIV and the other didn’t, and checked whether any new infections came from the partner on treatment. In the PARTNER study of gay couples, 782 couples reported condomless anal sex 76,088 times while the partner with HIV had a suppressed viral load. There were no HIV transmissions within those couples. The authors concluded that the risk in this situation is effectively zero.

The CDC’s review of the evidence draws on several couples studies like this one, and the WHO now states that people on treatment with an undetectable viral load will not transmit HIV to their sexual partners.

How long it takes, and staying there

  • Getting there. Reaching an undetectable level can take up to six months of treatment. Until a test confirms it, use condoms or PrEP (pre-exposure prophylaxis, HIV prevention medicine for the partner without HIV).
  • Staying there. Undetectable depends on taking treatment consistently. The CDC suggests viral load tests every 4 to 6 months, so people with HIV know they are still undetectable.
  • Knowing your partner’s result. U=U relies on your partner’s actual, recent viral load result, not on guesswork. If you’re not sure, it’s fine to ask them.

For comparison, receptive anal sex with a partner who has HIV and is not on effective treatment carries an estimated 1.4% (about 1 in 72) chance of HIV per act. With an undetectable partner, the sexual risk is gone.

Where U=U applies, and where it doesn’t

  • Sex. U=U covers anal, vaginal and oral sex, with or without a condom.
  • Pregnancy and birth. According to the CDC, an undetectable viral load also prevents HIV passing to a baby during pregnancy, labor and delivery.
  • Breastfeeding. Treatment lowers the risk a great deal, but the CDC says the risk through breastfeeding is not zero. Parents with HIV should plan infant feeding with their care team.
  • Sharing needles. U=U has not been established for needle-sharing. The CDC says an undetectable viral load likely reduces the risk, but by how much is unknown. Use new, sterile equipment every time. See HIV risk from sharing needles.
  • Other STIs. U=U is about HIV only. Treatment for HIV doesn’t stop chlamydia, gonorrhea, syphilis or other infections, so condoms and regular STI testing still matter. See HIV and STIs.

When you don’t know your partner’s status

If you don’t know whether a partner has HIV, U=U can’t tell you much on its own. Many people with HIV are on treatment and suppressed. Worldwide, UNAIDS reports that 93% of people on treatment were virally suppressed in 2023. But some people with HIV don’t know they have it, and others aren’t on treatment yet. The calculator takes this into account when your partner’s status is unknown.

What to do now

If you had sex in the last 72 hours with a partner who has HIV and isn’t on treatment, or whose viral load you’re unsure of, ask about PEP (post-exposure prophylaxis) now. PEP has to start within 72 hours, ideally within 24. See how to get PEP.

  • Your partner is undetectable and has recent results. Sex with them does not put you at risk of HIV. You may still want regular HIV and STI checks as part of routine care.
  • You’re not sure about their viral load. Talk with them, and with a clinician if you’re worried. If the exposure was recent, don’t wait on PEP.
  • You want ongoing protection with partners of unknown status. PrEP is very effective. Read about PrEP and PEP.
  • You’d like a test. Find free testing at gettested.cdc.gov, and check when each test is accurate.