What Activities Are of High and Low Risk for HIV?

Estimating HIV Transmission Risk by Exposure Type

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Whenever discussing HIV risk, it's important to first establish the four conditions that must take place in order for HIV transmission to occur:

  1. There must be body fluids in which HIV can thrive. This includes semen, blood, vaginal fluids or breast milk. HIV cannot thrive in the open air or in parts of the body with high acid content (such as the stomach or bladder).
  2. There must be a route of transmission by which body fluids are exchanged. Primary routes of transmission include certain sexual activities, shared needles, occupational exposure, or transmission from mother to child.
  1. There must be a means for the virus to reach vulnerable cells inside your body. This can occur through a rupture or penetration of the skin or through mucosal tissues of the anus or vagina. HIV cannot penetrate intact skin.
  2. There must be sufficient levels of virus in the body fluids. This is why saliva, sweat and tears are unlikely sources for HIV since the concentration of virus in these fluids is considered insufficient. Neutralizing enzymes in saliva (called secretory leukocyte peptidase inhibitors, or SLPIs) are known to greatly diminish HIV's ability to thrive.

Determining whether an activity is "high risk" or "low risk" is, therefore, dependent upon how efficiently an activity satisfies each of these four conditions.

HIV Transmission Can Occur After Only One Infection

Assigning an actual percentage to the "riskiness" of a certain activity is a tricky business. While statistics may suggest that there is only a one-in-200 (or 0.5%) chance of getting infected by such-and-such activity, that doesn't mean you can’t get infected after only one exposure.

Instead, a 0.5% "per-exposure" risk is meant to indicate that an average of one infection will occur out of 200 people who engage in a particular activity. It doesn't mean that mean you need to do something 200 times in order to get infected.

It's important to remember that risk estimates are based on two factors and two factors alone: that one person has HIV and the other doesn't.

Additional co-factors (such as co-existing STDs, general health, and the infected person's viral load) can further compound risk until an low risk activity is suddenly considerly higher.

Estimated HIV Transmission Risk Per Exposure

The estimates below should not be considered definitive but rather serve as as means to understand the relative risk of HIV by exposure type. The numbers are based on a meta-analysis of several large-scale studies which looked specifically at per-exposure risk.

ExposureExposure TypePer-Exposure Risk
AnalReceptive anal sex with ejaculation1.43% (one in 70)
 Receptive anal sex without ejaculation0.65% (one in 154)
 Insertive anal sex, uncircumcised0.62% (one in 161)
 Insertive anal sex, circumcised0.11% (one in 909)
VaginalVaginal sex, female-to-male (high-income country)0.04% (one in 2500)
 Vaginal sex, male-to-female (high-income country)0.08% (one in 1250)
 Vaginal sex, female-to-male (low-income country)0.38% (one in 263)
 Vaginal sex, male-to-female (low-income country)0.3% (one in 333)
 Vaginal sex sex, asymptomatic HIV0.07% (one in 1428)
 Vaginal sex sex, late-stage symptomatic HIV0.55% (one in 180)
OralOral-penile (fellatio), receptive0% to 0.04% (one in 2500)
 Oral-penile (fellatio), insertive0% to 0.005% (one in 20000)
 Oral-anal (anilingus), either partnernegligible
 Oral-vaginal (cunnilingus), either partnernegligible
PercutaneousShared injecting drug use, non-disinfected0.67% (one in 149)
 Occupational needlestick injury0.24% (one in 417)
 Non-occupational needlestick w/ discarded syringelow to negligible
 Blood transfusion (U.S.)0.0000056% (one in 1.8 million)
PregnancyMother-to-child, no antiretroviral therapy (ART)25% (one in four)
 Mother-to-child, ART two weeks prior to delivery0.8% (one in 125)
 Mother-to-child, on ART with undetectable viral load0.1% (one in 1000)

Reducing Your Personal HIV Risk

The purpose of understanding relative risk is to establish the means by which to reduce your personal risk of infection or the risk of transmitting HIV to others.

Oftentimes, it takes little to mitigate risk. For example, the consistent use of condoms correlates to a 20-fold decrease in HIV risk, while choosing insertive fellatio over insertive anal sex results in a 13-fold decrease. Conversely, the presence of an STD or genital ulcer increases the risk of HIV by anywhere from 200% to 400%.

Arguably the most important factor is assessing the likelihood of HIV transmission is the infected person's viral load.

Current data suggests that an HIV-infected person with an undetectable viral load is 96% less likely to transmit HIV than a person with detectable virus. The strategy, called Treatment as Prevention (TasP), strongly supports the use of antiretroviral therapy to reducing the infectivity of an HIV-positive person. It also reinforces the need for early testing to mitigate risk in mixed-status (serodiscordant) couples.

Knowing your serostatus and that of your partner allows you to make informed choice on how to better protect yourselves—whether it be to abstain from high risk activities, use condoms, or explore pre-exposure prophylaxis (PrEP) as a means to reduce the HIV-negative partner's susceptibility to infection.


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