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 0.12% per coital act, or about once per 800 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] Notes: *There is some controversy about transmission efficiency through oral sex. Campo and colleagues report no transmissions after 19,000 events reported by 135 discordant heterosexual couples. On the other hand, Wood and colleagues emphasize data are not adequate to estimate the risk, suggesting it could be higher than estimated rates in the above table.
Preventing HIV from sex
With more HIV testing, millions of couples in Africa are learning of HIV infection in one or both partners. 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.
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. For non-regular partners, condoms protect. 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 taking antiretrovirals for pre-exposure prophylaxis (PrEP) — before and after sexual contact — can cut their risk to get HIV by as much as 90%.
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8. Wood LF, Chahroudi A, Chen H-L, et al. The oral mucosa immune envirnment and oral transmission of SIV/HIV. Immunol Rev 2013; 254. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3821196/pdf/nihms477081.pdf (accessed 28 January 2019).
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