Anal Sex and Risk of HIV

The risk of human immunodeficiency virus (HIV), a virus that can be a sexually transmitted infection (STI), is quite high during unprotected anal sex (penetrating the rectum).

The anal sex risk is greater than vaginal sex, and greater for a receptive (bottom) partner when the penis is placed in the anus.

This article discusses the increased risk of anal sex for a receptive partner (though the risk exists for a "top" partner too). It presents a number of techniques for reducing this elevated risk of HIV.

Reasons for Increased Risk

The well-documented reasons for the increased risk that attaches to anal sex, and specifically the bottom partner, include:

• 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

Virus in the insertive ("top") partner's semen and pre-seminal fluid increases the risk of the receptive partner's infection with the rise in the HIV viral load (the amount of detectable virus). Some studies suggest route of HIV acquisition and sexual role can influence set point viral loads.

The HIV risk for a top partner can be increased as well by blood exposure from damaged rectal tissues during unprotected sex. This gives the virus a transmission route through the urethra and tissues that line the head of the penis (particularly under the foreskin).

HIV Risk Per-Act and Per-Partner

The per-act risk of HIV transmission is based on each sex act. Receptive anal sex presents a much greater risk than other sexual encounters including oral sex, and each act (of any type) adds up over time.

The Centers for Disease Control and Prevention (CDC) offers data on the per-act risk of HIV that varies across different types of sexual encounter. These risks include:

• Receptive (bottom) anal sex: 138 per 10,000 exposures (1.38%)
• Insertive (top) anal sex: 11 per 10,000 exposures (0.11%)
• Receptive vaginal sex (person with a vagina): 8 per 10,000 exposures (0.08%)
• Insertive vaginal sex (person penetrating a vagina): 4 per 10,000 exposures (0.04%)
• Oral sex: Risk is low to negligible

The per-partner risk is another way to evaluate HIV transmission. When HIV infections first began to emerge, and HIV testing was new, researchers learned that the single greatest factor in risk was the number of sex partners with whom one had unprotected receptive anal sex. That risk ranged from 29% with no recent partners to 85% with three or more partners.

Studies on per-partner HIV risk consider a number of variables, including the sex role, whether one or both partners is HIV-positive, the viral loads involved, and access to treatment among them.

Reducing Risk with Condoms

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

This is achieved with consistent condom use for protected sex. There are different ways to measure how effective condom use can be in protecting against HIV transmission, exposure per act and per partner among them. Studies have consistently found condom use to be an effective measure.

Condoms also can be used alongside other measures. It remains important, even when people have reduced the HIV transmission risk through medication. The CDC reports only 59.8% of Americans with HIV are able to achieve an undetectable viral load in the infected partner.

To this end, the consistent use of condoms can prevent 7 out of 10 transmissions through anal sex, according to the CDC.

Medications That Prevent Transmission

Treatment as prevention (TasP) is a strategy used to limit HIV spread by treating the infected partner. This means their viral load is reduced and they may achieve undetectable levels.

One study of 1,770 same-sex and heterosexual mixed-status couples found not a single case of HIV transmission, despite engaging in condomless anal or vaginal sex, when antiretroviral therapy (ART) is used.

There are now more than 40 ART drugs available, with different types of drugs in six classes of medication. Some drugs are available as daily combination medications. These ART drugs include:

• Entry-Attachment inhibitors, like Rukubio (fostemsavir) tablets or injectable Fuzeon (enfuvirtide)
• Integrase inhibitors, like Vocabria (cabotegravir) tablets
• Nucleoside reverse transcriptase inhibitors (NRTIs) like Viread (tenofovir) tablets
• Protease inhibitors (PIs) including Lexiva (fosamprenavir)
• Pharmacokinetic enhancers like Norvir (ritonavir) to be taken with PI
• Capsid inhibitors like Sunlenca (lenacapavir)

The use of pre-exposure prophylaxis (PrEP), whereby the uninfected partner takes daily medication like Apretude (cabotegravir), can also reduce risk. That said, condoms remain an important strategy for limiting HIV transmission (and other STIs) during anal sex with an HIV-positive partner.

Summary

Anal sex doesn't cause HIV, but people who have receptive anal sex are at higher risk for HIV infection when sex with an infected partner is not protected by condom. The insertive partner also is at risk, and condoms remain an important (and successful) strategy for preventing HIV transmission during anal sex.

Medications are also an important therapy. With ART drugs, an infected partner's viral load can be reduced or achieve undetectable levels, limiting the risk to the uninfected partner. PrEP can help to keep an uninfected partner healthy, while PEP may be able to prevent HIV infection after an exposure if it's started soon enough.

Be sure to discuss HIV prevention and anal sex transmission risks with your healthcare provider, who can assist with any prevention strategies, diagnosis, or treatment that may be needed.