In Africa, Europe, and the US, researchers have followed heterosexual couples in which one of the partners was infected with HIV, asked them about their sexual behaviour, and observed how many partners became infected. From these studies, the estimated average risk to contract HIV through vaginal sex without a condom with an HIV-infected partner is 0.05% to 0.11% per coital act, or about once per 900 to 2,000 coital acts (see table, below). Generally, less than 10% of HIV-positive men or women with HIV-negative spouses transmit HIV to their spouses in a year, even with continued unprotected (without condom) sex.
Table: Estimated risks to get HIV from an HIV-infected sex partner
Exposures (without a condom)
|Anal intercourse, receptive
|Anal intercourse, insertive
|Oral sex, receptive
|Oral sex, insertive
Sources: references [1-6]
Many factors influence the efficiency of sexual transmission. For example: When someone has a new infection, semen or vaginal fluids are have more virus than after a few months. Risk increases an estimated 2-4 times if either partner has a genital ulcer or sexually transmitted disease (STD). In addition, risk appears to vary from person to person for reasons no one understands.
If a man is HIV-positive, unprotected anal sex (penis in anus) is much more dangerous for the woman than vaginal sex (see table, above).
Preventing HIV from sex
Methods to prevent HIV transmission through sex are changing over time, and some methods may not be available everywhere. We encourage you to consult other sources for the latest and local advice.
Preventing HIV from spouses and other long-term partners: With more HIV testing, millions of couples in Africa are learning of HIV infection in one or both partners. While an HIV-positive test is bad news, knowing that a husband or wife is HIV-positive allow couples to take care of the HIV-positive partner and to protect the HIV-negative partner and new children.
Discordant couples can protect the HIV-negative partner by getting antiretroviral treatment (ART) for the HIV-positive partner and checking from time-to-time to see if the viral load is suppressed. ART reduces transmission risk by more than 90%. Discordant couples can also use condoms. An option for couples who want to get pregnant is pre-exposure prophylaxis (PrEP) with antiviral drugs for the HIV-negative partner. Couples with HIV in one or both spouses will be able to live, birth, and raise children like anyone else.
Preventing HIV acquisition from non-spousal partners: The risk to contract HIV from a non-spousal sexual partner depends on the risk associated with the specific sexual acts (see table, above) and on the risk a partner is HIV-positive. With more testing, including self-testing, regular non-spousal partners can see if anyone is HIV-positive, so they know if they need to take precautions. For non-regular partners, good advice to avoid getting HIV is summarized in a simple acronym – ABC: Abstain or Be faithful to one partner; if not, use a Condom. In addition, treating any STDs that you have reduces your risk to get HIV if you have unprotected sex. Our one caveat is that injections to treat STDs may be risks for HIV, so you should take care to ensure safe STD treatment (see Injection section).
Preventing HIV for men who have sex with men (MSM): MSM are at high risk to get HIV from receptive anal sex (see table). When MSM are both insertive and receptive partners, they can both get HIV and pass it on to other men through anal sex, leading to high percentages of MSM with HIV infection. Condom use can stop HIV transmission through anal as well as vaginal sex. Men taking ART are much less infective. HIV-negative men can take antiretrovirals for pre-exposure prophylaxis — before and after sexual contact — reducing their risk to get HIV by as much as 90%.
1. CDC. Antiretroviral postexposure prophylaxis after sexual, injection drug-use, or other nonoccupational exposure to HIV in the United States: recommendations from the U.S. Department of Health and Human Services. MMWR 2005; 54 (No. RR-2): 1-20.
2. Gray RH, W awer MJ, Brookmeyer R, et al. Probability of HIV-1 transmission per coital act in monogamous, heterosexual, HIV-1-discordant couples in Rakai, Uganda. Lancet 2001; 357: 1149-1153.
3. Mastro TD, Kitayaporn D. HIV type 1 transmission probabilities: estimates from epidemiological studies. AIDS Res Hum Retroviruses 1998 (suppl 3): S223-S227.
4. Varghese B, Maher JE, Peterman TA, et al. Reducing the risk of sexual HIV transmission: quantifying the per-act risk for HIV on the basis of choice of partner, sex act, and condom use. Sex Transm Dis 2002; 29: 38-43.
5. Vittinghoff E, Douglas J, Judson F, et al. Per-contact risk of human immunodeficiency virus transmission between male sexual partners. Am J Epidemiol 1999; 150: 306-311.
6. DeGruttola V, Seage GR III, Mayer KH, Horsburgh CR Jr. Infectiousness of HIV between male homosexual partners. J Clin Epidemiol 1989; 42: 849-856.
7. Rottingen J-A, Cameron DW, Garnett GP. A systematic review of the epidemiologic interactions between classic sexually transmitted diseases and HIV: how much is really known? Sex Transm Dis 2001; 28: 579-587.