HIV risk levels for the insertive and receptive partner in different types of sexual intercourse

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Since vaginal, anal and oral intercourse provide different types of biological pathways for HIV transmission, they carry varying levels of risk for both the insertive and receptive partner.

Vaginal intercourse

Globally, the most frequent route of sexual HIV transmission is unprotected vaginal intercourse.1 This is also the behaviour most often prosecuted in HIV-related criminal cases (see Criminal HIV Transmission blog at www.criminalhivtransmission.blogspot.com). There are differences in biological risk of HIV acquisition for men and women, and observed differences in that risk for men and women in high-income countries compared with those in low-income countries. In high-income countries, a woman is about twice as likely to acquire HIV from an HIV-positive male partner as compared with a man's relative risk of acquiring HIV from an HIV-positive female partner. In low-income countries, the biological risk of acquisition appears to be greater for both men and women, and relatively similar between the sexes. Some of the reasons behind these differences are explained below.

Risk of HIV transmission via vaginal intercourse, per sexual act

 

Risk to female having sex with HIV-positive male

Risk to male having sex with HIV-positive female

High-income countries

0.08%

(1 in 1250)

0.04%

(1 in 2500)

Low-income countries

0.30%

(1 in 333)

0.38%

(1 in 263)

Summarised from Boily MC et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infect Dis 9: 118-29, 2009.

Risk to a female having sex with an HIV-positive male

A combined analysis of all studies of HIV-transmission risk undertaken to date in high-income countries has found that unprotected vaginal intercourse with an HIV-positive man who is chronically infected and not on antiretroviral treatment (ART) carries an estimated 0.08% risk of infection for the woman per sexual act. In other words, an average of one transmission is expected to occur for every 1250 instances of exposure. Combined data from studies undertaken in low-income countries have yielded a considerably higher estimated risk level of 0.30% for the woman per sexual act: here, an average of one transmission is expected to occur for every 333 instances of exposure.2

Risk to a male having sex with an HIV-positive female

In the combined analysis for high-income countries, the risk for a man who has unprotected vaginal intercourse with an HIV-positive woman who is chronically infected and not on antiretroviral treatment (ART) is estimated to be 0.04% per sexual act. In other words, an average of one transmission is expected to occur for every 2500 instances of exposure. Combined data from studies in low-income countries indicate a much higher risk level, 0.38%: here, an average of one transmission is expected to occur for every 263 instances of exposure.2

Why are there differences between men and women, and between people in high-income and low-income settings?

Women are thought to be at a greater risk than men of acquiring HIV during vaginal intercourse for the following reasons:

  • The exposed surface area of the vagina is larger than that of the penis

  • Vaginal intercourse causes minute tears in the vaginal lining

  • The tissue of the vaginal lining contains certain types of cells that HIV can easily enter

  • Semen remains in the vagina for a prolonged period.

HIV can also pass through the skin of the penis, and certain factors (see Important factors affecting risk, below) can increase the likelihood of this happening during vaginal intercourse.

Researchers are uncertain about how to account for the differences observed between the risks of vaginal transmission in high-income versus low-income countries. One possibility is that less accurate estimates came from low-income countries (e.g., due to the under-reporting of high-risk behaviours in the studies that took place there). However, since there are many factors affecting risk levels, they also suggest that any number of as-yet-undetermined viral, biological and genetic factors may be implicated in these differences.2

Anal intercourse

Anal intercourse can be practised between two men or between a man and a woman. Despite its often taboo status impeding research into anal sex between men and women, there is evidence that a significant minority of heterosexuals have anal sex on a regular basis in both high-income3 and low-income settings.4

Unprotected anal intercourse carries a higher risk of sexual HIV transmission than unprotected vaginal intercourse. Although either sexual partner can acquire HIV from the other during unprotected anal intercourse, HIV is more likely to pass from an HIV-positive insertive partner to his receptive partner than from an HIV-positive receptive partner to his or her insertive partner.

The most widely cited study of per-act anal-transmission risk was published in 1999. It found that amongst men in high-income countries, unprotected anal intercourse with an HIV-positive insertive partner carried an estimated 0.82% risk of infection for the receptive partner per sexual act, or a 1-in-122 risk of transmission.5 It estimated the risk for the insertive partner as 0.06% per sexual act, or a 1-in-1666 risk of transmission.5

A more recent study amongst men from Sydney, Australia,6 has been able to more accurately estimate the per-act risks based on a wider variety of factors.  The data support a recent meta-analysis of all previous studies of the per-act risk of receptive anal intercourse to ejaculation for both heterosexuals and sex between men, which was estimated to be 1.4%.7 Table 5.2 below, provides an overview of the Australian study's estimates.

Risk of HIV transmission via anal intercourse, per sexual act

 

Per-act probability

Insertive partner's risk (circumcised)

0.11%

(1 in 909)

Insertive partner's risk (uncircumcised)

0.62%

(1 in 161)

Receptive partner's risk (without ejaculation)

0.65%

(1 in 154)

Receptive partner's risk (with ejaculation)

1.43%

(1 in 70)

Source: Jin F et al. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. AIDS, published online ahead of print, 2010.

Although the risk for the insertive partner in anal intercourse has previously been considered to be much lower than the risk for the receptive partner, this study suggests that for a man who is not circumcised, the risks of insertive intercourse are broadly similar to the risks of receptive intercourse without ejaculation.

Few studies have examined the risk of heterosexual unprotected anal sex. For an HIV-negative woman the per-act risk of anal intercourse is thought to be around ten times higher than the per-act risk for vaginal intercourse.8 There are no reliable data regarding the risk for the insertive HIV-negative man of anal intercourse between a man and a woman, but it is likely that the per-act risk is similar to sex between men.2

Oral sex (fellatio, cunnilingus, anilingus)

Oral sex can mean fellatio (mouth-penis sex); cunnilingus (mouth-vagina sex); or anilingus (mouth-anus sex). Fellatio is the only type of oral sex that carries more than a theoretical risk of HIV transmission, although the receptive partner in fellatio (the person who takes the partner’s penis into his or her mouth) is still much less likely to acquire HIV than the receptive partner in anal or vaginal intercourse. The risk for the insertive partner in fellatio is thought to be virtually nonexistent.

Researchers disagree, however, about the receptive partner’s exact risk level, with estimates ranging from zero risk (based on epidemiological studies amongst heterosexuals) to a 1-in-2500 risk of HIV infection (based on case reports amongst men who have sex with men). The uncertainty partly reflects the fact that most people who perform fellatio also engage in other higher-risk sexual activities; HIV transmission in those situations cannot therefore be attributed to fellatio in particular.

The receptive partner’s HIV risk level in oral sex is generally thought to be higher if he or she has bleeding gums or has cuts, sores or other abrasions inside the mouth, including those caused by dental procedures. However, there is no definitive evidence regarding such factors.

Researchers have identified several likely reasons for the relatively much lower risk of acquiring HIV from fellatio than from other forms of sex. An enzyme in saliva inhibits HIV; the low salt concentration of saliva may also inhibit HIV transmission; and tissues in the mouth and throat appear to be less susceptible to HIV than are genital or anal tissues. It is also probable that even if semen is swallowed, digestive enzymes can destroy HIV in the stomach.

References

  1. UNAIDS 2008 report on the global HIV/AIDS epidemic. Joint United Nations Programme on HIV/AIDS (UNAIDS), Geneva. (www. unaids.org/en/KnowledgeCentre/HIVData/GlobalReport/2008, 1-357), 2008
  2. Boily MC et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infect Dis 9(2): 118-129, 2009
  3. Halperin DT Heterosexual anal intercourse: prevalence, cultural factors, and HIV infection and other health risks, Part I. AIDS Patient Care STDS 13(12): 717-30, 1999
  4. Kalichman S et al. Heterosexual anal intercourse among community and clinical settings in Cape Town, South Africa. Sex Transm Infect 85(6): 408-10, 2009
  5. Vittinghoff E et al. Per-contact risk of human immunodeficiency virus transmission between male sexual partners. American Journal of Epidemiology 150: 306-311, 1999
  6. Jin F et al. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. AIDS, published online ahead of print, 2010
  7. Baggaley R et al. HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention. International Journal of Epidemiology, 1-16: e-published April 20, 2010
  8. Halperin DT et al. High level of HIV-1 infection from anal intercourse: a neglected risk factor in heterosexual AIDS prevention. 14th International AIDS Conference, Barcelona, abstract ThPeC7438, 2002
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.