Putting a Number on It: The Risk From an Exposure to HIV

Differences in Risk

Information on how risky certain types of unprotected sex are compared to others may help people make more informed decisions about the type of sex they are having.

Based on the meta-analysis estimates, we can draw several conclusions:

• Receptive anal sex carries a much higher risk of HIV infection than receptive vaginal sex.
  Research shows that the risk of HIV transmission from receptive anal sex is up to 18 times higher than from receptive vaginal sex.

• Receptive anal sex is riskier than insertive anal sex.
  Research suggests the risk of HIV transmission from receptive anal sex is 3 to 23 times higher than from insertive anal sex.

• Receptive vaginal sex is riskier than insertive vaginal sex.
  The risk from receptive vaginal sex is about twice as high as that from insertive vaginal sex.

• It is unclear exactly how much less risky oral sex is compared to vaginal and anal sex.

Conclusion

Although it's impossible to provide a client with their exact risk of HIV transmission from an exposure, some studies have managed to estimate an average risk for different types of sex. It's important to provide clients with additional information to help them interpret the findings. Here are some key messages:

1. These numbers
   • are challenging to calculate and should therefore be considered rough estimates
   • do not represent the risk of transmission from all exposures to HIV
   • represent the average risk of transmission in the absence of biological factors that can increase risk (such as STIs and a high viral load)
   • are most relevant to people in stable monogamous serodiscordant relationships
2. These numbers may seem low but
   • HIV transmission can occur after a single exposure
   • the risk may be much higher if certain biological risk factors, such as STIs or a high viral load, are present
   • as more exposures to HIV occur, the overall risk of transmission increases
   • most HIV transmissions in Canada occur through unprotected anal and vaginal sex
3. There are several ways of reducing the risk of HIV transmission from an exposure, such as post-exposure prophylaxis (PEP), using antiretroviral treatment to reduce viral load, circumcision, treatment for STIs and vaginal conditions, or engaging in lower-risk activities.
4. There is no way to reduce the risk of HIV transmission to zero after an exposure occurs. Taking measures to avoid an exposure in the first place (for example, through the correct use of condoms or other barrier methods, or by ensuring a partner has the same HIV status) can help reduce the overall risk of HIV transmission.

James Wilton is the project coordinator of the Biomedical Science of HIV Prevention Project at CATIE. James has an undergraduate degree in Microbiology and Immunology from the University of British Columbia.

References

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