HIV in Canada: A primer for service providers


Per-act Risk of HIV Transmission

Key Points

  • Certain activities generally carry a greater risk of HIV transmission than others.
  • Receptive anal sex is associated with the highest risk of HIV transmission, followed by injecting drugs using a shared needle.
  • Each exposure to HIV is unique and carries its own unique risk for HIV infection, based on many factors.

Several research studies have attempted to calculate the risk of HIV transmission associated with one exposure to HIV through sexual contact or from sharing needles. These studies have found that certain activities generally pose a greater risk of HIV transmission than others.

The following table shows estimates for the average transmission risk from one exposure to HIV through different activities, listed from highest to lowest risk:

 Activity (sexual activities are according to position of the HIV-negative partner)

Average risk estimate

Rate of transmission

 Receptive anal sex


1 transmission per 71 exposures

Injecting drugs using a shared needle 0.63% 1 transmission per 159 exposures

 Insertive anal sex


1 transmission per 909 exposures

 Receptive vaginal sex


1 transmission per 1,250 exposures

 Insertive vaginal sex


1 transmission per 2,500 exposures

It is important to understand that these per-act risk estimates are based on research studies that look into average rates of transmission. They do not consider the effect of specific biological factors that can impact risk, such as the viral load of the person with HIV. Because risk involves multiple factors, it is very difficult to accurately quantify the risk from one exposure. Each exposure to HIV has a unique risk of transmission, determined by the complex interplay of the various biological factors involved.

Receptive anal sex (where an HIV-negative person receives a penis into their anus), carries the highest risk of HIV transmission. Research suggests the risk may be 10 to 20 times higher than that for vaginal sex or insertive anal sex (where an HIV-negative person inserts their penis into an anus). This is partly because the epithelial cell layer lining the rectum is only one layer thick, making it more susceptible to tearing and inflammation, which can facilitate HIV transmission. In contrast, the epithelial cell layers in the vagina, penis and mouth are several layers thick.

Studies suggest that receptive vaginal sex is approximately two times riskier than insertive vaginal sex. This is partly because the mucous membranes of the vagina and cervix have a greater surface area than the urethra and foreskin of the penis, and the vagina remains in contact with sexual fluids for a longer time.

There is little to no risk of HIV transmission through oral sex. There is no risk of an HIV-negative person contracting HIV from receiving oral sex. There is a theoretical risk of HIV transmission when an HIV-positive male ejaculates in the mouth of an HIV-negative partner, however, the risk is still very low, much lower than for anal or vaginal sex. There are several biological reasons for this. Saliva contains enzymes that can inactivate HIV, the mouth and throat have multiple layers of epithelial cells, and fluids that contain HIV do not generally stay in prolonged contact with possible routes of infection after oral sex.

When a person is exposed to HIV through sharing needles, the risk of transmission is higher than the risk from vaginal and insertive anal sex. This relatively high risk is because the virus is introduced directly into the body, bypassing some of the body’s natural HIV defences.


Putting a number on it: the risk from an exposure to HIVPrevention in Focus

HIV Risk Behaviors – U.S. Centers for Disease Control and Prevention


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