‘Negligible’ risk of HIV transmission with a suppressed viral load, Canadian public health officials tell prosecutors

The potential for sexual HIV transmission when a person living with HIV adheres to antiretroviral therapy and maintains an undetectable viral load is ‘negligible’, says the Public Health Agency of Canada, in a review conducted for the country’s Department of Justice. Moreover, the risk of transmission is ‘low’ when a person living with HIV is taking antiretroviral therapy without an undetectable viral load of less than 200 copies/ml, condoms are used, or both.

It is hoped that this confirmation of the scientific evidence will help curtail the criminalisation of non-disclosure, which is widespread in Canada. People can be prosecuted for not disclosing their HIV-positive status when they engage in sexual activity that poses a “realistic possibility” of HIV transmission.

The Public Health Agency of Canada conducted a systematic review, in order to identify existing reviews and studies which provided data to calculate an absolute risk of sexual transmission of HIV between sexual partners of different HIV statuses.


person years

In a study “100 person years of follow-up” could mean that information was collected on 100 people for one year, or on 50 people for two years each, or on ten people over ten years. In practice, each person’s duration of follow-up is likely to be different.


In HIV, refers to the act of telling another person that you have HIV. Many people find this term stigmatising as it suggests information which is normally kept secret. The terms ‘telling’ or ‘sharing’ are more neutral.


In HIV, usually refers to legal jurisdictions which prosecute people living with HIV who have – or are believed to have – put others at risk of acquiring HIV (exposure to HIV). Other jurisdictions criminalise people who do not disclose their HIV status to sexual partners as well as actual cases of HIV transmission. 

systematic review

A review of the findings of all studies which relate to a particular research question and which conform to pre-determined selection criteria. 

absolute risk

The chance that a person will experience a specific event during a period of time. It is always between 0 and 1 (when expressed as a probability), or between 0 and 100 (when expressed as a percentage).

Studies published up to April 2017 were included. As the authors acknowledge, this means that some of the most important recent data, from both the PARTNER 2 and Opposites Attract studies, are not included. As no HIV transmissions occurred in people with undetectable viral loads in these studies, adding their data in would simply add to our certainty with which we can say that HIV transmission does not occur in these circumstances.

The review authors looked at various scenarios.

The HIV-positive sex partner is taking antiretroviral therapy and has a suppressed viral load. This estimate was based on the older data from the PARTNER and Opposites Attract studies, in which viral loads below 200 copies/ml were confirmed at least every six months. No HIV transmissions occurred: 0 transmissions over 1327 person-years (pooled incidence 0.00 transmissions/100 person- years, 95% CI 0.00–0.28). The authors say that if the more recent data from PARTNER 2 and Opposites Attract were included, the upper confidence interval of the estimate for would be lowered (from 0.28 to 0.13), but the point estimate of 0.00 would not change.

The authors describe the risk of transmission in these circumstances as “negligible”.

The authors looked for data on condom use in these circumstances, but did not find any additional studies. If either partner did also use condoms, the risk is also described as “negligible”.

The HIV-positive sex partner is taking antiretroviral therapy (with varying levels of viral load). Some HIV transmissions did occur in the studies which were included: 23 phylogenetically linked HIV transmissions across 10,511 person-years of follow-up (pooled incidence 0.22 transmissions/100 person-years, 95% CI 0.14–0.33).

The authors note that people in these cohorts generally had high levels of adherence to treatment and high levels of viral load suppression. Therefore, the potential for transmission in these circumstances may have been underestimated.

The HIV-positive sex partner is taking antiretroviral therapy (with varying levels of viral load) and either partner uses condoms. There was little evidence to inform this estimate. The authors rely on a 2012 systematic review which modelled the combined effect of antiretroviral therapy and condoms to derive per-act risks, ranging from 0.003 transmissions per 1000 acts (95% CI 0.00– 0.03) for insertive vaginal sex to 0.11 transmissions per 1000 acts (95% CI 0.02–0.73) for receptive anal sex. This risk is described as “low”.

Either partner uses condoms, without the HIV-positive partner taking antiretroviral therapy. The authors believe the conclusions of a 2012 Cochrane Collaboration review still provide the best evidence on this question. This found that among serodiscordant couples who reported “always” using condoms, there were 1.14 HIV transmissions per 100 person-years (95% CI 0.56–2.04). The Canadian authors describe this risk as “low”.

The Canadian Department of Justice used the conclusions of this study to inform a report on the justice system’s response to HIV nondisclosure which was published a year ago. This stated: “The criminal law should not apply to persons living with HIV who have engaged in sexual activity without disclosing their status if they have maintained a suppressed viral load (i.e., under 200 copies per ml of blood), because the realistic possibility of transmission test is not met in these circumstances.”

Moreover, “Unprotected sex with an HIV positive person who has not disclosed their status can no longer be considered to establish a prima facie case of HIV non-disclosure as evidence of treatment and viral load will always be relevant to determining whether the realistic possibility of transmission test is met.”

However, this important first step by the federal government towards limiting unjust HIV criminalisation does not constitute prosecutorial guidelines, which still need to be implemented at both the federal and provincial levels.


LeMessurier J et al. Risk of sexual transmission of human immunodeficiency virus with antiretroviral therapy, suppressed viral load and condom use: a systematic review. CMAJ 2018 November 19;190:E1350-60. doi: 10.1503/cmaj.180311