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People often want to know what their risk of HIV transmission is, and how much different HIV prevention strategies can reduce their risk. We have come a long way in our understanding of risk and how prevention strategies affect risk. Service providers can help clients to better understand and assess their risk, but must do so with the understanding that a person’s behaviours are only part of the picture. Understanding a person’s HIV risk requires the consideration of many individual, behavioural and contextual factors. While personal decisions and actions (such as using condoms) can affect someone’s risk of getting HIV, other social and structural factors (such as relationship power dynamics, unstable housing or lack of income) can also play a role in shaping their vulnerability to HIV. Explaining risk in a meaningful way can be complex and challenging. However, understanding the many factors that make up risk for HIV can not only help people assess their own risk but also find ways to better support the people that service providers work with.

What is risk and why is it important?

Risk refers to the chance or probability that something will happen. In the context of HIV prevention, risk is the chance that specific activities or events will result in HIV transmission. Did you know that no activity has a 100% risk of HIV transmission? Risk is all about uncertainty and it does not happen in a vacuum. It is influenced by many different factors that can change over time. Broadly, the following factors can affect a person’s HIV risk:

  • whether they participate in types of activities that carry a risk of HIV transmission (e.g., having certain types of sex, sharing injection drug use equipment)
  • how frequently they participate in these activities (e.g., how frequently they have sex without using an effective HIV prevention method, how frequently they share injection drug use equipment)
  • whether they have biological factors that can increase the risk of HIV transmission (e.g., a sexually transmitted infection)
  • whether they have other personal factors, such as mental health issues or substance use, that can affect their risk in a variety of ways, such as by affecting their judgment or choice-making and their ability to navigate consent
  • whether they have access to and choose to use different HIV prevention strategies (e.g., HIV treatment to maintain an undetectable viral load, condoms, pre-exposure prophylaxis [PrEP] and post-exposure prophylaxis [PEP])
  • social and structural factors (i.e., forms of oppression that create health inequities such as racism and homophobia)

It is important for service providers to have a solid understanding of all the factors that affect the chance of HIV transmission, and it is important to be able to discuss these factors with clients. Service providers can help clients understand and assess their HIV risk so they have the information they need to make informed decisions about the activities they participate in and their use of HIV prevention strategies. Furthermore, understanding that HIV risk is produced and reinforced through unfair differences in health status (i.e., health inequity) caused by social and structural factors can help service providers to better meet the holistic needs of their clients (e.g., supporting them to access other services including counselling or housing supports) and advocate for broader systemic changes (e.g., policy changes).

Caution with the concept of risk

Certain populations are disproportionately affected by HIV

In Canada, certain populations have disproportionately high rates of HIV that are concentrated in marginalized groups and communities.1,2 The populations that are disproportionately impacted by HIV in Canada include gay, bisexual and other men who have sex with men (gbMSM); two-spirit people; transgender people; Indigenous peoples (First Nations, Inuit and Métis); African, Caribbean and Black communities; and people who use drugs. However, this does not mean that being a member of one of these populations is a “risk factor” for HIV. Rather, it means that other factors are contributing to increased risk at a population level. In Canada, populations that have high rates of HIV disproportionately experience a range of social and structural forms of discrimination and exclusion (e.g., racism, homophobia, transphobia) that influence their social determinants of health (e.g., homelessness, poverty, social isolation) and their ability to access health services, leading to health inequities. In the context of HIV, health inequities in these populations include increased vulnerability to HIV and poorer health outcomes for people living with HIV.

These disparities can also create conditions that enable HIV to spread more rapidly in the population, which further increases health inequity. The greater the number of people who are living with HIV in a given population, the more likely it is that a member of that population will be exposed to HIV. For example, a man who has sex with men would have a statistically higher risk of getting HIV than a man who has sex only with women, even if both men are having the same types of sex, because of the higher prevalence of HIV among gbMSM.

Avoid shame and blame in conversations about risk

The concept of risk is often used directly or indirectly to place blame on individuals for activities they participate in. It is important to be aware of this when discussing HIV risk with clients. Labelling specific activities as “risky” or telling people that they “shouldn’t do” specific things can reinforce the experience of oppression and exclusion for them. Additionally, this approach does not acknowledge that the activities that can lead to HIV transmission are sometimes a result of factors beyond a person’s control that limit their choices. For example, they may have to share drug use equipment because new equipment is not available, or there may be a power imbalance in a sexual relationship that determines their use of HIV prevention strategies.

Furthermore, activities that can transmit HIV (like sex and drug use) are normal and common — we do them because they feel good, because they’re fun, because they can contribute to our overall well-being and for many other reasons. Conversations about risk should include exploring the benefits that people gain from the things they do. Sex-positive and drug-positive discussions around risk reduction are also important for reducing potential stigma associated with activities that can transmit HIV.

Finally, when talking about activities that can transmit HIV, it is important to recognize that everyone has the right to decide what activities they feel comfortable participating in and what benefits of these activities they weigh against their risks. People can be supported to make choices to reduce or even eliminate their risk of HIV transmission, but the choice to use a certain prevention strategy is up to them. People will have different levels of risk tolerance and different perceptions of their own risk for HIV and other sexually transmitted and blood-borne infections (STBBIs) like hepatitis C. Some people are willing to accept more risk than others: an individual’s risk tolerance is often influenced by multiple factors including their beliefs and their life experiences.

A tale of two risks: absolute and relative

The rest of this article will walk through some ways that risk can be considered and communicated to clients, using the concepts of absolute risk and relative risk. It’s important to understand that when it comes to talking about risk of HIV transmission, some things are uncertain. However, one thing that is certain is that undetectable equals untransmittable (U=U). This means that a person living with HIV who is on HIV treatment and maintains an undetectable viral load will not pass HIV through sex.3,4 Overall, the lower the viral load, the lower the chance of HIV transmission; when someone has an undetectable viral load the risk of sexual transmission is zero. Service providers should clearly communicate this fact to our clients to help them understand their sexual HIV risk and to promote widespread confidence in the U=U message.

With this in mind, service providers can give their clients more information about their individual risk of HIV transmission. Clients generally want to know two things about their risk:

  1. their risk of getting HIV or passing HIV (also known as absolute risk)
  2. how much a risk factor or prevention strategy can change their risk (also known as relative risk)

Absolute risk

In the context of HIV transmission, absolute risk can refer to the risk from one specific act (a single exposure to HIV) or it can refer to the risk of transmission over time (multiple exposures to HIV).

The risk of HIV transmission from a single exposure to HIV depends on many factors, making it difficult to estimate a person’s individual risk. Researchers have managed to estimate the average risk of HIV transmission from one exposure to HIV, through different types of exposures, by pooling data from multiple studies (known as meta-analysis).5 The risk estimates shown in the table below are average estimates based solely on the activity. They do not consider any other factors that might increase or decrease risk such as sex with a person who has an undetectable viral load or the use of PrEP. In fact, these studies were conducted before we fully understood the impact of having an undetectable viral load and before PrEP was widely available.

Risk of HIV transmission from different types of exposures


Estimate from meta-analysis5

Chance of getting HIV per act

Receptive anal sex


1 in 72

Insertive anal sex


1 in 909

Receptive vaginal sex


1 in 1250

Insertive vaginal sex


1 in 2500

Oral sex

Estimated to be much lower than any other type of sex, but unable to calculate

Unable to give an estimate

Needle-sharing injection drug use


1 in 159

The table above shows that the average risk of HIV transmission through one act of receptive anal sex with a person who is HIV positive is estimated to be 1.38%.5 This means that on average there will be 1 transmission for every 72 exposures to HIV through receptive anal sex. Since this is an average risk estimate it includes sexual acts involving people with both very high and low viral loads. Recall that risk through sex can be as low as zero if a person living with HIV is on treatment and has an undetectable viral load.

When numbers are used to explain risk, it is sometimes difficult for people to understand what they mean, or they might even misinterpret them. However, data from research is often what underpins our understanding of risk. What these numbers help us understand is that, on average, some activities have a higher or lower chance of HIV transmission than others, per act. For example, when it comes to sex, the activity that has the highest chance of HIV transmission is receptive anal sex. While vaginal sex (both receptive and insertive) has a lower risk than anal sex, it is still considered to be an activity with a high risk for HIV transmission.

An activity is considered high risk when there is a large amount of evidence for HIV transmission happening via this route. In other words, it is a common way that HIV is transmitted when no effective prevention method is used. The table below uses qualitative terms to express the level of risk associated with different types of exposures. Note that even though each of these activities has a different numerical estimate of risk, all of them (except for oral sex) are considered high risk.

Risk of HIV transmission from different types of exposures (when no effective prevention method* is used) in qualitative terms


Level of risk

Receptive anal sex

High risk

Insertive anal sex

High risk

Receptive vaginal sex

High risk

Insertive vaginal sex

High risk

Giving oral sex

Little to no risk

Receiving oral sex

No risk

Needle-sharing injection drug use

High risk

*Highly effective HIV prevention strategies include using condoms, taking PrEP, taking PEP, taking treatment to maintain an undetectable viral load and using new needles and other equipment when injecting drugs.

Qualitative expressions such as high risk, low risk, very low risk and no risk are often used to describe the level of risk associated with different activities. Qualitative expressions can be easier to communicate than numerical expressions and may reflect the risk of HIV transmission in a way that is more clear or meaningful to a client. However, they are not very precise. Qualitative terms can still be a good starting point for a conversation about risk, before layering in information about other factors that can change the baseline risk.

Risk can also increase over time. A person’s overall risk of getting HIV increases the more they are exposed to HIV. Like the one-time risk of HIV transmission from a specific exposure, the risk over time is also unique to each person. This risk depends on how many times a person is exposed to HIV — which, in turn, depends on factors such as how often the person engages in an activity that can transmit HIV, the level of risk associated with the activity, the chance that they are exposed to HIV with each act and how consistently they are using a prevention strategy.

In HIV prevention, risk assessments tend to focus on a person’s risk from a single exposure to HIV and not their risk over time. This may lead people to underestimate their risk because risks that may be considered small in the short term can add up and become larger in the long term. For example, the average risk of HIV transmission from one act of unprotected receptive vaginal sex is estimated to be 0.08%.5 Although a person might view this risk as low, the risk will grow with more acts of unprotected vaginal sex. After 100 exposures, the cumulative risk of HIV transmission becomes about 8%. Then, consider that the risk could be higher if certain biological factors are present (for example, if one partner has a sexually transmitted infection [STI]) or the risk could be lower if HIV prevention strategies are used. This brings us to relative risk.

Relative risk

Each activity where HIV can be transmitted carries a particular level of absolute risk. An individual’s absolute risk depends on the type of activities they participate in and how often. However, as mentioned above, a number of additional factors can modify a person’s absolute risk. Data from research studies have helped us learn more about which factors can increase or decrease risk (risk reduction). Relative risk estimates tell us what factors can change a person’s risk and by how much.

Certain biological factors can increase the chance of HIV transmission.5 As previously noted, a key factor that can increase or decrease the risk of transmission is the amount of HIV in bodily fluids, which is known as viral load. The higher the viral load the greater the chance of HIV transmission, and the lower the viral load the lower the chance of transmission.6 When someone has an undetectable viral load their risk of HIV transmission to a sexual partner is zero.3,4 Other factors that can increase HIV transmission are genital ulcers5,7 (a symptom of certain STIs such as herpes or syphilis) and a vaginal condition called bacterial vaginosis.8,9 For example, genital ulcers can double the risk of HIV transmission.5

It is important for clients to know about the effect of these biological factors because there are ways to address them and it is possible to lower one’s risk even when they are present. For example, a person who is sexually active can get tested regularly for STIs so that if they develop an STI it can be identified and treated as early as possible. If a person has an STI, using an HIV prevention strategy such as PrEP can still lower the risk of HIV transmission.

Risk reduction

HIV prevention strategies can reduce the risk of HIV transmission. Relative risk estimates tell us which HIV prevention strategies are highly effective. The term “highly effective” means we can be confident that the risk is significantly reduced (and sometimes even eliminated) when the strategy is used consistently and correctly. There are many highly effective HIV prevention strategies:

In addition to using these strategies, people can choose to engage in types of sex with either low or no risk for HIV (e.g., oral sex, fingering, hand jobs, mutual masturbation) and they can choose to use drugs in ways that have low or no risk for HIV (e.g., inhaling, snorting, swallowing), although there may still be a risk for transmission of other kinds of infections and for drug poisoning or overdose.

The effectiveness of different HIV prevention strategies varies according to which strategy is used and how well it is used. For example, a highly effective prevention strategy that can be used by people living with HIV is taking HIV treatment as prescribed. When a person with HIV takes HIV treatment and maintains an undetectable viral load, their risk of transmitting HIV through sex is reduced by 100%.3,4 Their risk of transmitting HIV through the sharing of injection drug use equipment is also reduced; however, we don’t have a good estimate for how much.10

Another example is the use of PrEP by HIV-negative people. PrEP has been shown to reduce the risk of getting HIV through sex by more than 99%11,12 when taken as prescribed. However, there is less research on PrEP use by people who inject drugs. In the single study that has looked at this population, the risk reduction was estimated to be 84% among those who used PrEP consistently compared with those who did not.13 Therefore, it’s possible that PrEP is more or less effective depending on the route of HIV transmission, although the data are limited. In addition, PrEP requires consistent use of prescription medications, taken as prescribed. If doses are missed, the level of protection can be affected.

Condoms provide a barrier that HIV cannot pass through; however, they still do not reduce risk for HIV by 100%. This is because they sometimes fail (i.e., they can break, slip or leak) and they are not always used correctly and consistently. Estimates from research on condom use among heterosexual people and gbMSM have produced wide-ranging estimates — from 69% to 94% risk reduction.14–19 This wide range may have to do with the limitations of observational research and the different ways in which researchers have conducted the analyses. The effectiveness of condoms will be maximized if they are used correctly and consistently every time a person has sex with a chance of passing HIV.

It is important to note that there can be many barriers to accessing and/or using HIV prevention strategies. For example, people without insurance coverage may have trouble accessing PrEP, PEP or HIV treatment. Some people may not be able to afford condoms or may not know where to get them for free. A host of other issues can prevent people from using or adhering to HIV prevention strategies including stigma, relationship power imbalance, mental health issues and prioritizing basic needs over HIV prevention.

Absolute risk revisited

While people want to know how much a prevention strategy can reduce their risk, they also want to know their overall absolute risk of HIV transmission while using a given strategy. The relative risk estimates for HIV prevention strategies allow us to compare effectiveness between people who use the strategy and people who do not. They tell us how much the risk has changed (e.g., the risk has increased or decreased considerably), but they do not tell us what the new level of risk actually is.

It’s simplest to use qualitative terms to express a client’s overall absolute risk because numbers can be difficult to interpret when there are so many factors to consider. When any highly effective strategy is used consistently and correctly, the absolute risk for HIV transmission ranges from zero to very low.

Let’s look at an example for anal sex and PrEP:

Baseline absolute risk: When no highly effective HIV prevention strategy is used, anal sex carries a high risk for HIV transmission.

Relative risk: Taking PrEP is highly effective at reducing the risk of HIV transmission. It can reduce the chance of sexual transmission by more than 99% when taken consistently and correctly.

Overall absolute risk : When a person is using PrEP as an HIV prevention strategy for anal sex their risk of getting HIV is extremely low. In other words, it is very rare for a person to get HIV while using PrEP as prescribed even when they are engaging in a high-risk activity.

Communicating risk clearly and supporting clients to reduce their risk for HIV

Communicating risk can be challenging and there are a lot of factors to consider, but a basic understanding of the concepts explored in this article can help service providers better support their clients to assess and address their HIV risk.

Here are some suggestions and reminders for communicating risk and supporting clients to reduce their HIV risk:

  • Risk is all about uncertainty, and few things in life are 100% certain. Risk is not static and can change on the basis of many individual, behavioural and contextual factors. Help clients understand the many factors that affect risk.
  • Having open and non-judgmental discussions about the risks and benefits of certain activities with clients can help to reduce stigma and help them to choose HIV prevention strategies that will work for them.
  • There are many highly effective prevention strategies that can reduce or eliminate the chance of getting or passing HIV. Note that their effectiveness depends on how well they are used. Service providers can support clients to access HIV prevention strategies and to use them consistently and correctly.
  • Each individual has their own comfort level when it comes to risk and their own assessment of the value of different benefits. As a result, one person might weigh the risks and benefits involved in HIV risk assessment differently than another.
  • Service providers can help support clients to address social determinants of health that affect their vulnerability to HIV. Service providers can provide linkage to local services that address other needs such as housing supports, mental health care, primary care, access to nutritious food and more.
  • Everyone has the right to unbiased information about HIV risk and all available HIV prevention options. People also have the right to access health services that respect their choices, their activities and their identities.


Seven Ways to Prevent HIVclient resource

Safer Sex Guideclient resource

Pre-exposure prophylaxis (PrEP) - fact sheet

Post-exposure prophylaxis (PEP) - fact sheet

PrEP to Prevent HIV: Your Questions Answeredclient resource

PEP: Preventing HIV after a potential exposure – client resource


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About the author(s)

Camille Arkell is CATIE’s manager, harm reduction, HIV prevention and testing. She has a Master of Public Health degree in Health Promotion from the University of Toronto and has been working in HIV education and research since 2010.

Externally reviewed by: Dr. Michael Montess & Ryan Lisk