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.
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.
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.
No comments have been made.