What Is the Risk of HIV From Anal Sex?

The risk of human immunodeficiency virus (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, which doubles the risk of infection with every one-log (one digit) rise in the person's viral load (the amount of virus able to be detected in a person who's infected)

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-act risk of HIV through condomless anal sex was roughly around 1.4% for the receptive ("bottom") partner.

The risk of transmission was further increased if the insertive partner was uncircumcised (0.62% uncircumcised vs. 0.11% 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 10 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 engage in both receptive and insertive anal sex without condoms have a summary risk of 39.9%.
• An HIV-negative partner who engages solely in insertive anal sex without condoms has a summary risk of 21.7%, while the HIV-negative receptive partner has a risk of around 40.4%.

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

The effectiveness of the strategy known as treatment as prevention (TasP, taking HIV medication to prevent transmission), is evidenced by the PARTNER1 and PARTNER2 studies in which not a single HIV infection occurred among 1,770 same-sex and heterosexual mixed-status couples despite engaging in condomless anal or vaginal sex.

The studies, which ran from 2010 to 2018, showed without a doubt that undetectable equals untransmittable in a real-world setting.

The use of pre-exposure prophylaxis (PrEP), whereby the uninfected partner is prescribed a daily dose of the HIV drug Truvada (emtricitabine and tenofovir), can also reduce risk. Studies have shown that when taken daily, PrEP reduces the risk of getting HIV from sex by about 99%. 

Apretude (cabotegravir extended-release injectable suspension) is another newer PrEP option. It is given as an injection administered every two months to the uninfected partner and has been shown to greatly reduce infection risk.

Although these figures may suggest that condoms are no longer needed, neither TasP nor PrEP can prevent other sexually transmitted infections (STIs).

Moreover, according to the Centers for Disease Control and Prevention (CDC), only 59.8% 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 7 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 or condomless anal sex, there are medications that can reduce your risk of infection, referred to as 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.