In Africa, Europe, and the US, scientists 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.i ii iiiivvvi
Many factors influence the efficiency of sexual transmission between men and women. Risk is greater when the HIV-positive partner has a very new infection; in early infections, semen or vaginal fluids are likely to have more virus than at other times. Some evidence shows that male circumcision reduces men’s risk to get HIV from women, but increases women’s risk to get HIV from men. Risk increases an estimated 2-4 times if either partner has a genital ulcer or sexually transmitted disease (STD).vii 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 acquisition from spouses and other long-term partners
With more HIV testing, including near-routine testing of pregnant women as part of programs to prevent mother-to-child transmission, 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.
If your spouse or someone else tests HIV-positive, this does not tell you anything about their sexual behaviour. Many HIV-positive husbands and wives deny having other partners. And virtually all who had other sex partners had possible blood exposures as well. It’s not possible to say how anyone got HIV without tracing and testing sex partners, and testing others who visited the same clinics, dentists, hairdressers, etc.
Similarly, knowing that you and your spouse have had no outside sexual risks does not tell you anything about your HIV status. Either of you may be HIV-positive from blood exposures. If you are both HIV-negative and want to stay that way, you need to work with you spouse to limit not only sexual risks, but also blood exposures.
Discordant couples can protect the HIV-negative partner by using condoms with all penetrative sex. Recent research shows that HIV transmission can be dramatically reduced if the HIV-positive partner starts antiviral treatment. Another option for couples who want to get pregnant is pre-exposure prophylaxis with antiviral drugs for the HIV-negative partner. It is reasonable to expect that these options will improve over time, so that couples will soon be able to live with HIV as a chronic but not fatal disease that rarely transmits to spouses or children.
Preventing HIV acquisition from casual and commercial partners
The risk to contract HIV from a sex worker or non-regular sexual partner depends on the risk associated with the specific sexual acts (see table, above) and on the risk that a partner is HIV-positive. Good advice to avoid getting HIV from non-regular partners 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).
Risks for men who have sex with men
Men who have sex with men (MSM) have high risk for 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.
i 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.
ii Gray RH, Wawer 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.
iii Mastro TD, Kitayaporn D. HIV type 1 transmission probabilities: estimates from epidemiological studies. AIDS Res Hum Retroviruses 1998 (suppl 3): S223-S227.
iv 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.
v 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.
vi DeGruttola V, Seage GR III, Mayer KH, Horsburgh CR Jr. Infectiousness of HIV between male homosexual partners. J Clin Epidemiol 1989; 42: 849-856.
vii 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.