HIV risk from vaginal sex

For women

CDC says either partner can get HIV during vaginal sex, and that HIV can enter the body through the tissue that lines the vagina and cervix. For a woman having vaginal sex once without a condom with a man who has HIV and isn’t on treatment, the estimate is 0.08% (about 1 in 1,200).

Most partners don’t have HIV, so with a man of unknown status the chance is much smaller. It depends heavily on where you are:

• United States: 0.00014% (about 1 in 730,000)
• United Kingdom: <0.0001% (about 1 in 3,700,000)
• South Africa: 0.0023% (about 1 in 44,000)

The calculator also asks whether a male partner of unknown status is circumcised, because circumcised men are less likely to have acquired HIV in the first place.

For men

For a man having vaginal sex once without a condom with a woman who has HIV and isn’t on treatment, the estimate is 0.04% (about 1 in 2,500). With a woman of unknown status, it varies by country:

Circumcision

Large trials found that circumcised men are less likely to get HIV through vaginal sex. The calculator lowers a circumcised man’s estimate by 50%, so one act with a woman who has HIV becomes 0.02% (about 1 in 5,000).

Circumcision protects the man, not his partner. A trial of men who already had HIV found circumcision didn’t reduce transmission to their female partners. See circumcision and HIV.

Pregnancy and after birth

Pregnancy and the months after giving birth raise a woman’s risk of getting HIV through vaginal sex. With a partner who has HIV and isn’t on treatment, the per-act estimate is 0.23% (about 1 in 440) during pregnancy and 0.32% (about 1 in 310) after birth. If you’re pregnant and think you’ve been exposed, tell your clinician quickly. They can advise on PEP (post-exposure prophylaxis) and testing. See HIV risk in pregnancy.

Other STIs

Infections that cause sores or inflammation make HIV easier to pass on. The calculator multiplies the estimate by 2 if either partner has one. For a woman with a partner who has HIV, that makes one act 0.16% (about 1 in 620). Read more about HIV and STIs.

What lowers the risk

• Condoms. About 80% lower risk in real-world use. A woman having sex with a condom with a partner who has HIV: 0.016% (about 1 in 6,200).
• PrEP. PrEP (pre-exposure prophylaxis) is medicine that people without HIV take to prevent it. Taken as prescribed, it lowers the risk from vaginal sex by about 90%. A woman on consistent PrEP with a partner who has HIV: 0.008% (about 1 in 12,000). Daily pills and long-acting injections are options for women and men. CDC says pills reach maximum protection for receptive vaginal sex after about 21 days of daily use.
• Treatment. A partner with HIV who keeps an undetectable viral load doesn’t pass HIV on through sex. See U=U.
• No ejaculation. The calculator lowers a woman’s estimate by 50% when her partner didn’t ejaculate inside her.

What to do now

1. Consider PEP first if you’re within 72 hours. It’s taken daily for 28 days, and you don’t need a test result before starting.
2. Test after the window period. A lab antigen/antibody test (blood from a vein) can detect HIV from 18 days, and a negative result is conclusive from 45 days. If you take PEP, expect follow-up tests about 4 to 6 weeks and 12 weeks after the exposure. See test accuracy by week.
3. Check for other STIs at the same visit, since they can be passed on during the same act.
4. Think about the future. If you have sex without condoms with partners whose status you don’t know, ask about PrEP.