What Is the Risk of HIV From Anal Sex?

Identifying the Means to Mitigate Risk in Heterosexual and Same-Sex Partners

Ellen Denuto/Getty Images

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, researcher 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% 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% uncircumcised vs.

0.11% circumcised).

By comparison, the per-partner risk—in which an HIV-positive person is in an exclusive relationship with a 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 was able to suggest that:

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

(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% and 75%. 

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 ten 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.


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.

Smith, D.; et al. "Condom efficacy by consistency of use among MSM: US" 20th Conference on Retroviruses and Opportunistic Infections (CROI); March 2-6, 2013; Atlanta, Georgia; abstract 32.

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].

Continue Reading