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, which allow the virus direct access into the bloodstream through tiny tears or abrasions
  • The porousness of rectal tissues, providing access even when undamaged
  • The high concentration of HIV in semen and pre-seminal fluid ("pre-cum"), which doubles the risk of infection with every one-log (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).

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 condomless anal sex was roughly around 1.4 percent for the receptive ("bottom") partner.

The risk of transmission was further increased if the insertive partner was uncircumcised (0.62 percent uncircumcised vs. 0.11 percent circumcised).

By contrast, 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 gay or bisexual men and included neither the length of the relationship nor the frequency of condomless sex. An analysis of the data was able to show that:

  • Partners who engaged in both receptive and insertive anal sex without condoms have a summary risk of 39.9 percent.
  • An HIV-negative partner who engages solely in insertive anal sex without condoms has a summary risk of 21.7 percent, while the HIV-negative receptive partner has a risk of around 40.4 percent.

Strategies to Reduce Risk

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

Current evidence has shown that the consistent use of antiretroviral therapy (ART) in the HIV-infected partner completely eliminates the risk of HIV transmission when viral activity is suppressed to undetectable levels.

The strategy, known as Treatment as Prevention (TasP), is evidenced by the PARTNER1 and PARTNER2 studies in which not a single HIV infection occurred among 1,770 gay and heterosexual mixed-status couples despite engaging condomless anal or vaginal sex. The studies, which ran from 2010 to 2018, showed unequivocally that undetectable equals untransmittable in a real-world setting.

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 the risk by anywhere between 44 percent and 75 percent. 

Although these figures may suggest that condoms are no longer needed, neither TasP nor PrEP can prevent other sexually transmitted diseases. Moreover, according to the Centers for Disease Control and Prevention (CDC), only 59.8 percent of Americans with HIV are able to achieve an undetectable viral load. Without complete viral suppression, TasP is rendered useless, placing the uninfected partner at risk.

To this end, the consistent use of condoms can prevent seven out of 10 transmissions through anal sex, according to the CDC. This provides yet another layer of protection when used with other safer sex practices.

A Word From Verywell

If you believe you may have been exposed to HIV, either through a condom burst of condomless anal sex, there are medications that can reduce your risk of infection, referred to 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 possible, ideally within one to 36 hours of exposure.

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