Criminal Justice System's Response to Non-Disclosure of HIV

Part B: HIV Transmission Risks and Sexual Activity

PHAC has performed a comprehensive synthesis of the currently available medical science on sexual HIV transmissionFootnote 23 to determine the risk of sexual transmission between serodiscordant partners (i.e., one individual is HIV positive and the other is HIV negative), when one or more of the following are in place: HIV treatment (i.e., antiretroviral therapy or ART), a suppressed level of HIV in the blood (due to effective treatment), and/or male condom use.

Sexual transmission of HIV requires that: (1) there is a source of infection; (2) there is a means for transmission to occur; (3) there is a host who can get the infection; (4) there is a way for the virus to get to the cells in the host’s body; and, (5) there is enough virus delivered to the host to start an infection.Footnote 24 The Canadian AIDS Society (CAS) uses two factors to classify the level of HIV transmission risk associated with an activity: (1) whether transmission could occur based on the five conditions above; and (2) whether there is known evidence that transmissions have occurred due to the activity. These criteria are used to classify risks as:

no risk
(no potential for HIV transmission; no confirmed transmission);
negligible risk
(potential for HIV transmission though efficiency greatly diminished; no confirmed transmission);
low risk
(potential for transmission; few reports of transmission under specific conditions); and,
high risk
(potential for transmission; repeatedly associated with transmissions in scientific studies).Footnote 25

The following conclusions can be drawn from the 2017 evidence synthesis prepared by PHAC:

Sexual activity with an HIV positive person poses a negligible risk of transmission where that person is taking HIV treatment as prescribed and has maintained a suppressed viral load based on consecutive tests done every four to six months

In cases involving sexual activity between HIV-serodiscordant partners, the risk of HIV transmission is negligible where:

  • The partner living with HIV takes their treatment consistently, and has a viral load less than 200 copies per ml of blood (i.e., a suppressed viral load) on consecutive tests done every four to six months.
    • This is true whether a condom is used or not, and regardless of the sexual activity at issue;
    • Studies found zero transmissions under these conditions, though a statistical risk of 0.00-0.28 transmissions per 100 person-yearsFootnote 26 could not be ruled out.
Sexual activity with an HIV positive person poses a low risk of transmission where that person is on HIV treatment but has not achieved a suppressed viral load

In cases involving sexual activity between HIV-serodiscordant partners, the risk of HIV transmission is low where:

  • The partner living with HIV is taking treatment but does not have a suppressed viral load less than 200 copies per ml of blood on consecutive tests done four to six months apart.
    • This is true regardless of the sexual activity at issue;
    • Absolute risk is estimated to be 0.14-0.33 transmissions per 100 person-years;Footnote 27
    • Adding the use of a condom in this scenario is likely to give additional protection against HIV transmission, but the risk would still be classified as low;
    • There is insufficient evidence to estimate HIV transmission risk for oral sex when ART (with variable viral load) and condoms are used together.
Sexual activity with an HIV positive person poses a low risk of transmission where that person is not on HIV treatment but a condom is used

In cases involving sexual activity between HIV-serodiscordant partners, the risk of HIV transmission is low where:

  • The partner living with HIV is not taking treatment but condoms are used consistently during sex.
    • Absolute risk is estimated to be 0.56-2.04 transmissions per 100 person-years.Footnote 28

The United States Center for Disease Control and Prevention has estimated the risk associated with oral sex as follows:

Oral sex with an HIV positive person poses a low risk of transmission

The risk of oral sex (without a condom or use of ART) is low but non-zero (approximately 0-4 transmissions per 10,000 exposures).Footnote 29

  • While many studies have been unable to observe transmissions due to oral sex, there are confirmed reports of transmission in others.Footnote 30
  • Risks associated with oral sex are higher if the HIV positive individual is receiving oral sex, and if ejaculation occurs. When the tissue in the mouth of the HIV-negative individual is damaged (e.g., open cuts or sores, or recent dental work), the risk of transmission is also increased.Footnote 31

Important factors to consider:

  • A negligible risk does not mean zero risk; a theoretical possibility of transmission cannot be ruled out based on the scientific data.
  • In the studies examined, most participants on ART took their medication very consistently, and regularly had their viral loads tested, which allowed many of them to achieve and maintain a very low viral load. Taking medication consistently is extremely important for achieving and maintaining a viral load less than 200 copies per ml of blood.
  • When consecutive viral load tests done every four to six months show less than 200 copies per ml of blood, this likely means there is sustained viral load suppression. However, it is not possible to know the viral load on a continuous basis. It is therefore important for health care providers and people living with HIV to understand factors that may result in their viral load going back above 200 copies per ml of blood (referred to as virological failure), such as: not taking medication consistently or correctly, having a drug resistant virus, drug and/or alcohol abuse, and previous HIV medication history (e.g., types of medications taken in the past, and whether past treatments have been successful in reducing the viral load).Footnote 32
  • Latex condoms create an impermeable physical barrier to HIV and other STIsFootnote 33, but a potential for transmission remains due to breaks, slips, leaks, and other incorrect use that may occur; it is not possible to know prior to a sexual encounter whether perfect or correct use versus condom failure may happen.
  • The risks associated with oral sex under protective scenarios (e.g., when ART and/or condoms are used) are difficult to determine. However, they would not be expected to be higher than the risks for other acts.
  • Although sex is the primary mode of HIV transmission in CanadaFootnote 34, the risk of transmission for a single act (even without condoms or ART) is lower than is often thought. Although PHAC did not examine unprotected risks in their evidence synthesis, other groups have estimated risk for a variety of unprotected acts. According to modelled estimates, risk ranges from 4-138 transmissions per 10,000 sex acts, depending on the act (see Table 1 below), suggesting that HIV is transmitted relatively inefficiently by sexual means. These small risks for individual acts are however cumulative, such that risk rises with the number of acts performed.
Table 1. Modelled per-act risks of HIV transmission for unprotected (i.e., without condoms or ART) sexual acts
Sexual act HIV transmissions per 10,000 acts (95% confidence intervalTable note i)
Receptive anal intercourse 138 (102-186)
Insertive anal intercourse 11 (4-28)
Receptive vaginal intercourse 8 (6-11)
Insertive vaginal intercourse 4 (1-14)
Performing penile-oral intercourse Low but non-zero (0-4)
Receiving penile-oral intercourse Low but non-zero (0-4)

Adapted from: Patel P, Borkowf CB, Brooks JT, Lasry A, Lansky A, Mermin J. Estimating per-act HIV transmission risk: A systematic review. AIDS. 2014;28(10):1509-1519.

Table note i

The 95% confidence interval indicates the range within which we would expect the results to fall 19 times out of 20, if a study were repeated many times over.

Return to table note i referrer

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