What Is the Risk of HIV From Anal Sex?

Mitigating Risk in Heterosexual and Same-Sex Couples

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The risk of HIV through unprotected anal intercourse is seen to be extremely high, as much 18 times greater than vaginal intercourse. The reasons for the increased risk are well known and include such factors as:

  • The fragility of rectal tissues, allowing the virus a direct access into the bloodstream through tiny tears or abrasions.
  • The porousness of rectal tissues and their high susceptibility to infection even when undamaged.
  • The high concentration of HIV in semen and pre-seminal fluid ("pre-cum"), which can double the risk of infection with every one-log (i.e., one digit) rise in the person's viral load.

Furthermore, the secretion of blood from damaged rectal tissues can increase the risk for the insertive ("top") partner, providing the virus a route of transmission through the urethra and tissues that line the head of the penis (particularly under the foreskin).

Assessing HIV Risk "Per-Act" and "Per-Partner"

In their review of 16 different high-quality studies, researchers at the Imperial College and the London School of Hygiene and Tropical Medicine concluded that the per-risk act of HIV through condom-less anal sex was roughly around 1.4 percent for the receptive ("bottom") partner.

While the level of risk was not seen to vary by sexual orientation, whether heterosexual or men who have sex with men (MSM), the risk was seen to increase significantly if the insertive partner was uncircumcised (0.62 percent uncircumcised vs. 0.11 percent circumcised).

By comparison, the per-partner risk—in which an HIV-positive person is in an exclusive relationship with an HIV-negative partner—painted a somewhat clearer picture for both the receptive and insertive partners.

The ten studies reviewed were conducted only among MSM and included neither the length of the relationship nor the frequency of condom-less sex.

The analyses of the data were able to suggest that:

  • partners who engaged in both receptive and insertive anal sex without condoms had a summary risk of 39.9 percent.
  • an HIV-negative partner who engaged solely in insertive anal sex without condoms had a summary risk of 21.7 percent, while the HIV-negative receptive partner had a risk of around 40.4 percent.

(While the results of the meta-analysis are compelling, it’s important to note that the differing models for the reviewed studies do leave some room for interpretation.)

Strategies to Reduce HIV Risk from Anal Sex

As with any other mode of HIV transmission, prevention requires a combination of strategies to more effectively:

  • Reduce the infectivity of the HIV-positive partner.
  • Reduce the susceptibility of the HIV-negative partner.
  • Prevent HIV for coming into contact with vulnerable cells and tissues.

The use of combination antiretroviral therapy (ART) in the HIV-infected partner is known to significantly lower the risk of transmission when viral activity is fully and consistently suppressed to undetectable levels. The strategy, popularly known as Treatment as Prevention (TasP), has been shown to reduce HIV risk by as much as 96% within the construct of a mixed-status (serodiscordant) relationship.

Meanwhile, the use of pre-exposure prophylaxis (PrEP)—whereby the uninfected partner is prescribed a daily dose of the HIV drug Truvada—is known to decrease risk by anywhere between 44 percent and 75 percent. 

The consistent and proper use of condoms is also seen to be vital in preventing HIV infection, with the U.S. Centers for Disease Control and Prevention reporting that the consistent use of condoms can prevent seven out of 10 anal transmissions. (The same report also concluded that only one-in-six MSM reported consistent condom use.)

The data largely supports the combined use of TasP, PrEP, and condoms as the most effective means to minimize HIV risk, both in heterosexual and MSM partners engage in anal sex.

By contrast, condom-less sex is still not recommended, even if both TasP and PrEP are used, as there may be potential for long-term risk in some mixed-status couples.

Accidental Exposure and Anal Sex

If you believe you may have been exposed to HIV, either through a condom burst of condomless anal sex, there are medications that can greatly reduce the risk of infection called post-exposure prophylaxis (PEP).

PEP consists of a 28-day course of antiretroviral drugs, which must be taken completely and without interruption. In order to minimize the risk of infection, PEP must be started as soon as possibleideally within one to 36 hours of exposure.

Sources:

Baggaley, R.; White, R.; and Boily, C. "HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention." International Journal of Epidemiology. August 2010; 39(4):1048-1063.

Cohen, M.; Chen, Y.; McCauley, M.; et al. "Prevention of HIV-1 infection with early antiretroviral therapy." New England Journal of Medicine. August 11, 2011; 365(6):493-505.

Grant, R.; Lama, J.; Anderson, P.; et al. "Preexposure Chemoprophylaxis for HIV Prevention in Men Who Have Sex with Men." New England Journal of Medicine. December 30, 2010; 363(27):2587-2599.

Baeten, J.; Donnell, D.; Ndase, P.; et al. "Antiretroviral Prophylaxis for HIV Prevention in Heterosexual Men and Women." New England Journal of Medicine. August 2, 2012; 367(5):399-410.

Supervie, V.; Viard, J; Costagliola, D.; et al. "Heterosexual risk of HIV transmission per sexual act under combined antiretroviral therapy: systematic review and Bayesian modelling." Clinical Infectious Diseases. April 19, 2014; pii: ciu223 [Epub ahead of print].

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