HIV in Canada: A primer for service providers

Per-act Risk of Sexual HIV Transmission

Key Points

  • Certain sexual activities generally carry a greater risk of HIV transmission than others.
  • Receptive anal sex is associated with the highest risk of HIV transmission while oral sex is associated with the lowest.
  • Each exposure to HIV is unique and carries its own unique risk for HIV infection.

Several research studies have attempted to calculate the risk of HIV transmission associated with one act of unprotected sex (for example, if a condom or PrEP is not used) with a person who is HIV positive. These have found that certain types of sex 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 sexual activities:

 Sexual activity

Average risk estimate

Rate of transmission

 Receptive anal sex

1.4%

1 transmission per 71 exposures

 Insertive anal sex

 0.11%

1 transmission per 909 exposures

 Receptive vaginal sex

 0.08%

1 transmission per 1,250 exposures

 Insertive vaginal sex

 0.04%

1 transmission per 2,500 exposures

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 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. In contrast, the epithelial cell layers in the mouth, vagina and penis are several layers thick.

Studies suggest that receptive vaginal sex is approximately two times riskier than insertive vaginal sex. This is partly because the vagina has a greater surface area than the penis and remains in contact with infected fluids for a longer time.

Oral sex is considered to carry a much lower risk than anal or vaginal sex. There are several biological reasons for this. Saliva contains enzymes that can inactivate HIV, the mouth and throat are covered by multiple layers of epithelial cells, and fluids that contain HIV do not stay in prolonged contact with possible routes of infection.

These comparisons of risk 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 viral load. Because risk involves multiple factors, it is very difficult to accurately quantify the risk from one act of unprotected sex with someone who is HIV positive. Each exposure to HIV has a unique risk of transmission, determined by the complex interplay of the various biological factors involved.

Resources

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

HIV transmission risk – Centers for Disease Control and Prevention

Sources

  1. Patel R, Borkowf CB, Brooks JT et al. Estimating per-act HIV transmission risk: a systematic review. AIDS. 2014;28(10):1509-19.
  2. Baggaley RF, White RG, Boily M-C. HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention. International Journal of Epidemiology. 2010 Aug;39(4):1048–63.
  3. Baggaley RF, Boily M-C, White RG, et al. Risk of HIV-1 transmission for parenteral exposure and blood transfusion: a systematic review and meta-analysis. AIDS London England. 2006 Apr 4;20(6):805–12.
  4. Baggaley RF, White RG, Boily M-C. Systematic review of orogenital HIV-1 transmission probabilities. International Journal of Epidemiology. 2008 Dec;37(6):1255–65.
  5. Boily M-C, Baggaley RF, Wang L, et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. The Lancet Infectious Diseases. 2009 Feb;9(2):118–29.
  6. Powers KA, Poole C, Pettifor AE, et al. Rethinking the heterosexual infectivity of HIV-1: a systematic review and meta-analysis. The Lancet Infectious Diseases. 2008 Sep;8(9):553–63.
  7. Wilton J. Putting a number on it: the risk from an exposure to HIV. Prevention in Focus. Fall 2012. Available from: http://www.catie.ca/en/pif/summer-2012/putting-number-it-risk-exposure-hiv