Service providers working in HIV prevention are often asked by their patients and clients about the risk of HIV transmission from an exposure to HIV through sex. What do the latest studies tell us about this risk? And how should we interpret and communicate the results?

Challenges in calculating a number

It isn’t easy for researchers to calculate the risk of transmission from an exposure to HIV through sex. To do this effectively, a group of HIV-negative individuals need to be followed over time and their exposures to HIV—both the number of times they are exposed and the types of exposure—need to be tracked. 

As you can imagine, accurately tracking the number of times a person is exposed to HIV is very difficult. Researchers ask HIV-negative individuals enrolled in these studies to report how many times they have had sex in a given period of time, what type of sex they had, how often they used condoms and the HIV status of their partner(s). Because a person may have trouble remembering their sexual behaviour or may not want to tell the whole truth, this reporting is often inaccurate.

Furthermore, a person does not always know the HIV status of their partner(s). For this reason, researchers usually enroll HIV-negative individuals who are in stable relationships with an HIV-positive partner (also known as serodiscordant couples). Researchers can then conclude that any unprotected sex reported by a study participant counts as an exposure to HIV.

Several studies have aimed to estimate the average risk of HIV transmission from a specific type of unprotected sex (for example, vaginal/anal/oral; insertive/receptive). Due to the difficulties of calculating this risk, these studies have produced a wide range of numbers. To come up with a more accurate estimate for each type of unprotected sex, some researchers have combined the results of individual studies into what is known as a meta-analysis.

All exposures are not equal

The results of several meta-analyses suggest that some types of sex carry on average a higher risk of HIV transmission than others. Below are estimates from a 2014 meta-analyses that have combined the results of studies conducted in high-income countries.

Anal sex

A meta-analysis exploring the risk of HIV transmission through unprotected anal sex was published in 2014.¹ The analysis estimated the risk through receptive anal sex (receiving the penis into the anus, also known as bottoming) to be 1.4%. (This means that an average of one transmission occurred for every 71 exposures.) The analysis estimated the risk for insertive anal sex (inserting the penis into the anus, also known as topping) to be 0.11%.

Vaginal sex

This 2014 meta-analysis also explored the risk of transmission through vaginal sex.¹ It estimated the risk of HIV transmission through receptive vaginal sex (receiving the penis in the vagina) to be 0.08% (equivalent to 1 transmission per 1,250 exposures). The analysis estimated the risk from insertive vaginal sex (inserting the penis into the vagina) to be 0.04% (equivalent to 1 transmission per 2,500 exposures).¹

Oral sex

No meta-analysis estimates exist for oral sex (vaginal or penile) because too few good-quality studies have been completed. This is because it is difficult to find people whose only risk of HIV transmission is unprotected oral sex. The 2014 review concluded that vaginal and penile oral sex pose a “low” risk of transmission but was unable to calculate a risk estimate.¹

Interpreting the numbers—what additional information needs to be provided?

Some clients may see these numbers and think their risk of HIV transmission is low. Therefore, caution is needed when interpreting them. If these numbers are provided to clients, they should be accompanied by information that helps shed light on why the risk may be higher than it seems.

Transmission can occur after one exposure.

It is important to emphasize that a person could become infected from having unprotected sex once or a person could have unprotected sex many times and not become infected, regardless of how low or high the risk per exposure is.

A risk of 1% would mean that an average of one infection would occur if 100 HIV-negative people were exposed to HIV through a certain type of sex. It does not mean that a person needs to be exposed 100 times for HIV infection to occur.

These are estimates of average risk in the absence of biological factors that increase risk.

The numbers in the table above are rough estimates. They are averages and do not represent the risk from all exposures to HIV through a certain type of sex.

We know that no two exposures to HIV are exactly the same. Research shows that, in addition to the type of sex that led to the exposure, several factors can increase or decrease the risk that an exposure to HIV leads to infection. The most important factor that can affect risk is the viral load of the person living with HIV. We know that if a person with HIV has an undetectable viral load they will not transmit HIV to a sex partner. However, if their viral load is high then the risk of HIV transmission increases. Other factors can also affect transmission risk such as the presence of sexually transmitted infections (STIs), a man being uncircumcised, activities that can cause tearing and inflammation, such as rough sex, longer sex, douching, enemas before anal sex, and potentially tooth brushing, flossing or dental work before oral sex. Each exposure to HIV carries a unique risk of transmission that depends on the type of sex and a combination of biological factors.

The more exposures, the greater the risk.

Although the risk of HIV transmission from a single exposure may seem low to some people, this risk increases over multiple exposures. In other words, a person who is exposed to HIV more often has a greater overall risk of HIV transmission than someone who is exposed less often.

If a woman has unprotected vaginal sex 100 times with a man who is HIV-positive, the cumulative risk is approximately 10% and may be higher if biological risk factors are present.

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,¹ 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 about 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 about 13 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. However, we know that the risk is much lower.

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 Pre-exposure prophylaxis (PrEP), post-exposure prophylaxis (PEP), using antiretroviral treatment to reduce viral load, condoms, 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 to HIV occurs. Taking measures to avoid an exposure in the first place can help reduce the overall risk of HIV transmission.

References

• 1. a. b. c. d. e. Patel P, Borkowf CB, Brooks JT, et al. Estimating per-act HIV transmission risk: a systematic review. AIDS. 2014 Jun 19;29(10):1509–10.