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Clients often want to know how great their risk of becoming infected with HIV or transmitting HIV is and how effectively different strategies can reduce this risk. Answering these questions in a way that is meaningful to clients can be complex and challenging. Frontline HIV prevention workers need to have a solid understanding of risk and how to help clients assess their risk in order to prevent new HIV transmissions.
Risk tells us about the possibility of harm. In the context of HIV prevention, it refers to the chance that specific activities or actions will result in HIV transmission.
Risk can generally be communicated to clients in two ways:
1. as a numerical expression. For example, the average risk of HIV transmission during one act of unprotected receptive anal sex is 1.4%; or antiretroviral treatment can reduce the risk of heterosexual HIV transmission by up to 96%.
2. as a qualitative expression. For example, the risk of HIV transmission during unprotected receptive anal sex is “very high”; or antiretroviral treatment can be “highly effective” at reducing the risk of heterosexual HIV transmission.
Accurately assessing and communicating risk is important because a client’s perceived risk of HIV transmission plays a major role in determining whether or not they will take measures to reduce their risk (by, for example, using condoms). Other factors that come into play when people make decisions about what precautions they will or will not take include how much they want to avoid HIV transmission and how much they feel unprotected sex increases their sense of pleasure and intimacy.
Risk is all about uncertainty. If a specific outcome is certain—0% or 100%—then no risk is involved. People often feel comfortable with certainties but with risk there are none.
When it comes to HIV transmission, uncertainties are unavoidable. Completely eliminating one’s risk of HIV transmission is not possible for many individuals because few sexual activities carry no risk of HIV transmission and few prevention strategies are 100% effective. At the same time, no activities carry a 100% chance of HIV transmission.
To complicate matters, people often have difficulty understanding probabilities and may interpret them in different ways. For example, “1.4%” or “high-risk” can mean different things to different people.
In addition, each individual has a different level of risk they are willing to take, which is often influenced by their attitudes and beliefs regarding risk, HIV and pleasure. Some people are more comfortable with a certain level of risk while others are more averse to taking risks.
Also, many factors and variables influence a person’s risk of HIV transmission, including the viral load of the HIV-positive partner, the kind of sex they are having (anal, vaginal or oral), whether he or she is the insertive or receptive partner, whether either partner has a sexually transmitted infection (STI), and any tearing and inflammation at the mucous membranes. As a result, every client’s risk of HIV transmission is unique. The more factors that are taken into account, the more accurate the risk estimate will be. Unfortunately, we don’t always know how much each one impacts a person’s overall risk of HIV transmission. Consequently, measures of risk tend to be generalizations and are always “best guesses.”
Clients generally want to know two things about their risk:
1. their risk of becoming infected with HIV or transmitting HIV (also known as their absolute risk); and
2. how much a risk factor or prevention strategy can change their risk (also known as relative risk).
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 an act of unprotected sex depends on a wide range of factors, it is difficult to estimate and assess someone’s individual risk. Some researchers have managed to estimate the average risk of HIV transmission from an exposure to HIV through specific types of sex.1,2 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%.
It is not easy for a person to use these numbers to assess their personal risk. It’s important to keep in mind that regardless of how low a percentage may seem, transmission can occur after a single exposure to HIV. Also, these numbers do not represent the risk from all exposures to HIV; they represent the average risk of HIV transmission in the absence of biological factors (such as other STIs and a high viral load) that can increase risk.
Many frontline service providers and transmission guidelines use qualitative expressions, such as “high-risk” or “low-risk,” to describe the level of risk associated with different activities. For example, the HIV Transmission Guidelines for Assessing Risk published by the Canadian AIDS Society (CAS) assign activities to one of four categories (no risk, negligible risk, low risk or high risk) based on two criteria: whether there is a potential risk of transmission and whether there is evidence of transmission. Qualitative expressions can be easier to communicate than numerical expressions and may reflect the risk of HIV transmission in a way that is more meaningful to the client.
However, the use of qualitative expressions has its disadvantages:
• Expressions such as “high-risk” and “low-risk” can be open to interpretation unless the criteria used to assign an activity to a category are clearly explained.
• These expressions don’t tell a client how high or low the risk is.
• Grouping multiple activities into a single risk category may not reflect some of the important differences in risk between activities within a category. For example, all types of unprotected anal and vaginal sex are normally considered high-risk activities but research shows that unprotected receptive anal sex carries a higher risk of HIV transmission than other types of unprotected vaginal and anal sex.
• This approach focuses on the activity that led to the exposure and often ignores biological factors, such as viral load, that can significantly increase or decrease the risk of HIV transmission. For example, clients may want to know if unprotected vaginal and anal sex are still high-risk activities when the viral load is undetectable.
Discussing with clients the absolute risk from an exposure to HIV may help them assess their risk and adopt strategies to reduce it (such as using condoms, lubrication, engaging in less risky types of sex, reducing their—or their partner’s—viral load, treating STIs, or using post-exposure prophylaxis.)
Risk over time
Similar to the risk of HIV transmission from a specific exposure, the risk over a given period of time is also 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 them, how consistently and correctly they are using condoms—and the unique transmission risk from each exposure that does occur.
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).
For example, the average risk of HIV transmission from one act of unprotected vaginal sex is estimated to be 0.1%. Although a client may consider this risk to be low, this risk will grow if they continue to have unprotected vaginal sex. After 100 exposures through unprotected vaginal sex, the cumulative risk of HIV transmission becomes approximately 10%. This risk could be even higher if certain biological risk factors are present, for example, if one partner has an STI or if the HIV-positive partner has a high viral load.
Exploring relative risk
Relative risk tells us about how much something, such as a risk factor or prevention strategy, can change a client’s risk. For example, acute HIV infection can increase the risk of HIV transmission by up to 2500% (a 26-fold increase);3,4 STIs and some vaginal conditions, such as bacterial vaginosis, can increase the risk of transmission by up to 700% (an 8-fold increase);5,6,7 or being on treatment and having an undetectable viral load can decrease the risk of heterosexual HIV transmission by up to 96% (a 26-fold decrease).8
Clients need to know about the factors that can increase or decrease their risk and by how much these factors can change their absolute risk.
Explaining how much a strategy can reduce risk (relative-risk reduction)
The effectiveness of different HIV prevention strategies and tools can vary greatly. Simply stating that a strategy reduces risk without saying by how much can be misleading. If a person overestimates the effectiveness of a strategy, they could potentially feel a false sense of security and, as a result, engage in more risky behaviours, thus increasing their overall risk of HIV transmission.
How much a strategy reduces risk is most often communicated as a percent change. For example, a recent study known as HPTN 052 found that antiretroviral treatment can reduce the risk of heterosexual HIV transmission by up to 96%.8
Risk reduction can also be communicated in other ways. For example, a 96% relative risk-reduction is equivalent to approximately a 26-fold decrease in risk. Since the percent change is generally a higher value than the fold or times change, communicating risk reduction as a percent may lead a person to overestimate the level of protection provided by a strategy.
It’s important for clients to understand that the relative risk reduction associated with a prevention strategy is not a static value, but can change depending on how well the strategy is used. For example, the ability of antiretroviral treatment to reduce the risk of HIV transmission will be much lower than 96% if a person does not adhere to their medications or if one partner has an STI. Similarly, when condoms are used consistently and correctly, they provide close to 100% protection; however, when they are used incorrectly and inconsistently, this level of protection drops.
Furthermore, the relative risk reduction calculated in a study may not apply to everyone. Clinical trials enroll specific populations, so the results may not be directly transferable to other populations. For example, the HPTN 052 study enrolled heterosexual serodiscordant couples almost exclusively and we don’t know if the results also apply to gay men and people who use injection drugs.
In addition, trials typically provide a comprehensive package of prevention services to participants—such as adherence and risk-reduction counselling, free condoms, regular STI testing and STI treatment—all of which may improve the protection provided by a strategy. In the “real world” outside of a clinical trial, the same prevention strategy may be less effective because the comprehensive package of services is not readily available.
Considering baseline risk
Clients not only want to know how much a strategy can reduce their risk, they also want to know their absolute risk of HIV transmission while they are using a strategy.
Relative risk numbers describe changes in risk but do not tell us what the risk is changed to. For example, we know that successful antiretroviral treatment can reduce the risk of heterosexual HIV transmission by up to 96%. This means that the risk has been reduced significantly compared to what it was initially; 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 what the actual risk is.
Consequently, it is difficult for clients to use relative risk information by itself to assess their risk of HIV transmission while using a prevention strategy. Assessing this risk requires that clients 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.
As a result of differences in baseline risk, it’s 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. For example, a person who has an STI will have a higher risk of HIV transmission while using a strategy than a person who is using the same strategy and has no STIs.
Simply because a strategy can significantly lower a client’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 adopting a prevention strategy that significantly reduces their risk. For example, we know that the HIV transmission risk from receptive anal sex is up to 18 times higher per exposure than the risk from vaginal sex.2 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.
Communicating risk can be challenging but a deeper understanding of the concepts explored in this article may help people working in HIV prevention provide a more thorough risk assessment for clients.
Below are some suggestions for assessing and communicating risk.
• If you use qualitative expressions, such as “high-risk” or “low-risk,” be clear about the criteria used to assign an activity to each category.
• If you provide numbers, make sure you explain what these numbers mean and how they can change as a result of biological risk factors.
• Be sure to explore the factors that influence a client’s risk of transmission from an exposure and over time. Explain that the overall risk of HIV transmission increases with every exposure.
• When communicating how well a prevention strategy will work for a client, discuss the factors that can make a strategy less effective. Also explore how a client’s baseline risk will influence their risk of HIV transmission while using a prevention strategy.
• Using visual aids and scenarios may help a client understand their risk.
• In addition to talking about HIV risk in terms of probabilities, you may want to also talk about how transmission occurs and about the epidemiology of HIV in Canada.
• Since perceived risk is only one factor that influences risk-taking decisions, it is also important to explore the other factors that may be playing a role in a client’s decision-making, such as their understanding of HIV and motivations for engaging in unprotected sex.
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2. a. b. Boily M-C, Baggaley RF, Wang L et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infectious Diseases. 2009 Feb;9(2):118–29.
3. Wawer MJ, Gray RH, Sewankambo NK et al. Rates of HIV-1 Transmission per Coital Act, by Stage of HIV-1 Infection, in Rakai, Uganda. Journal of Infectious Diseases. 2005 May 1;191(9):1403–9.
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6. Atashili J, Poole C, Ndumbe PM et al. Bacterial vaginosis and HIV acquisition: a meta-analysis of published studies. AIDS. 2008 Jul 31;22(12):1493–501.
7. Cohen CR, Lingappa JR, Baeten JM et al. Bacterial vaginosis associated with increased risk of female-to-male HIV-1 transmission: a prospective cohort analysis among African couples. PLoS Medicine. 2012 Jun;9(6):e1001251.
8. a. b. Cohen MS, Chen YQ, McCauley M et al. Prevention of HIV-1 infection with early antiretroviral therapy. New England Journal of Medicine. 2011 Aug 11;365(6):493–505.
About the author
James Wilton is the Project Coordinator of the Biomedical Science of HIV Prevention Project at CATIE. James has an undergraduate degree in Microbiology and Immunology from the University of British Columbia.