High vs. Low Risk Activities for HIV Transmission

When 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, healthcare exposure, or transmission from mother to child.
3. There must be a means for the virus to reach vulnerable cells inside the 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.
4. 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 the 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 Exposure

Assigning an actual percentage to the "riskiness" of a certain activity is a tricky business. While statistics may suggest that there is only a 1-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 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 sexually transmitted infections (STIs), general health, and the infected person's viral load, can further compound risk until a low-risk activity is suddenly considerably higher.

Estimated HIV Transmission Risk Per Exposure

The estimates below should not be considered definitive but rather serve as a 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.

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 STI or genital ulcer increases the risk of HIV by anywhere from 200% to 400%.

Arguably the most important factor in assessing the likelihood of HIV transmission is the infected person's viral load. Data suggest that the risk of a person living with HIV with an undetectable viral load transmitting the virus is essentially zero.

The strategy called treatment as prevention (TasP) strongly supports the use of antiretroviral therapy to reduce the infectivity of a person with HIV. 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 an 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.