HIV in Canada: A primer for service providers
Communication of Risk
- Accurate and meaningful communication of risk is important to help people understand their risk of HIV transmission and to make informed decisions.
- Absolute risk can refer to risk from one specific exposure or it can refer to the risk of transmission over a given period of time.
- Relative risk tells us how much something, such as a biological factor or prevention strategy, can change a person’s risk.
Accurate and meaningful communication of risk is important to help people understand their risk of HIV transmission and to make informed decisions. However, new knowledge of the factors that can increase the risk of HIV transmission, in addition to the availability of new highly effective HIV prevention strategies, has made the communication of risk more complex.
Risk can generally be communicated in two ways: as a numerical expression or as a qualitative expression. People generally want to know two things about their risk:
- They want to know their risk of becoming infected with HIV or transmitting HIV (also known as their absolute risk). For example, the average risk of HIV transmission during one act of unprotected receptive anal sex is 1.4%; the risk of HIV transmission during unprotected receptive anal sex is “very high.”
- They want to know how much a risk factor or prevention strategy can change their risk (also known as relative risk). For example, circumcision can reduce the risk of HIV transmission by up to 70% for heterosexual men.
Exploring Absolute Risk
Absolute risk can refer to risk from one specific exposure or it can refer to the risk of transmission over a given period of time.
Risk from a Single Act
Because the risk of HIV transmission from one act of unprotected sex (for example, if a condom or PrEP is not used) depends on a wide range of factors, it is difficult to estimate and assess someone’s particular risk. Some researchers have managed to estimate the average risk of HIV transmission from an exposure to HIV through specific types of sexual activity. For example, the average risk of HIV transmission through one act of unprotected receptive anal sex with a person who is HIV positive has been estimated to be 1.4%. However, these numbers represent the average risk of HIV transmission so they don’t account for biological factors (such as other untreated sexually transmitted infections) that can increase risk, or the use of highly effective prevention strategies that can decrease risk. It is also important to keep in mind that regardless of how low a percentage may seem, transmission can occur after a single exposure to HIV.
Risk over Time
Just as the risk of HIV transmission from a specific exposure is unique to that sexual encounter, so too is the risk over a given period of time unique to each individual. This risk depends on how many times a person is exposed to HIV — which, in turn, depends on how often a person is having sex, the chances that their partner(s) have a different HIV status than they do, and how consistently and correctly they are using an effective prevention strategy (for example, condoms or PrEP) — and the unique transmission risk from each exposure that occurs.
In HIV prevention, risk assessments tend to focus on a client’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 accumulate and became large in the long term. In other words, a client’s overall probability of HIV transmission increases the more they are exposed to HIV (a concept known as cumulative risk).
Exploring Relative Risk
Relative risk tells us how much something, such as a biological factor or prevention strategy, can change a person’s risk. These are most often communicated as a percent change. For example, circumcision can reduce the risk of HIV transmission by up to 70% for heterosexual men.
However, clients not only want to know how much a strategy can reduce their risk (relative risk), they also want to know their risk of HIV transmission while they are using a strategy (absolute risk).
Although relative risk numbers describe changes in risk, they do not tell us what the risk is changed to. For example, an interim analysis of a study known as HPTN 052 found that that HIV treatment reduces the risk of heterosexual HIV transmission by up to 96%. This means that the risk has been reduced significantly compared with its initial value; it does not mean that the risk has been reduced to 4%. In other words, relative risk is a comparison and does not say anything about the actual risk.
Consequently, it is difficult for people to use relative risk information by itself to assess their risk of HIV transmission while using a prevention strategy. Assessing this risk requires that people also have a good understanding of the context in which they are using the strategy and what their risk was to begin with, also known as their baseline risk.
It is possible for two people who are using the same risk-reduction strategy in the exact same way to have different absolute risks of HIV transmission, because of differences in their baseline risk. For example, a person who has inflammation in the genital or rectal tissues may have a higher risk of HIV transmission while using a particular strategy than a person using the same strategy who does not have inflammation.
Simply because a strategy can significantly lower a person’s risk does not necessarily mean that their risk while using the strategy will be low. If a person has a very high baseline risk, their risk may still be high after they adopt a prevention strategy that significantly reduces their risk. For example, we know that the HIV transmission risk from receptive anal sex is up to 20 times higher per exposure than the risk from vaginal sex. Even though a strategy may be able to reduce the risk of HIV transmission by the same amount for both types of sex, the absolute risk may still be higher through anal sex because it poses a higher baseline risk than vaginal sex. Similarly, a person who is using a certain risk-reduction strategy and is having sex 10 times a week may have a higher risk of HIV transmission than someone who is using the same strategy but is only having the same type of sex twice a week.
Certainly uncertain: Challenges in communicating HIV risk – Prevention in Focus
Views from the front lines: Communicating risk – Prevention in Focus
- Heise LL, Watts C, Foss A, et al. Apples and oranges? Interpreting success in HIV prevention trials. Contraception. 2011;83:10–5.
- Wilton J. Certainly uncertain: challenges in communicating HIV risk. Prevention in Focus. Summer 2012 issue. Available from: http://www.catie.ca/en/pif/summer-2012/certainly-uncertain-challenges-communicating-hiv-risk