HIV risk – where do perception and reality overlap?

Whereas pregnancy occurs quite frequently after unprotected sex, as discussed in the previous commentary, HIV is not transmitted so easily.  In their guidance on PrEP in 2015, WHO refers to substantial risk at a level of around 3% per year, which of course means that 97% of people in that risk group do not become HIV-positive in that year.  However, risk can only be measured at a group level.  Not only does this mean that there may be unrecognized risk factors, but also at the individual level we seldom calculate a mathematical risk of something happening to us.  So a better understanding of how people perceive their risk and how this relates to their actual likelihood of becoming HIV-positive is important for many aspects of HIV prevention and behaviour change communication.  Among gay men and other men who have sex with men in Europe, Australia and the US, self-identification, combined with a few screening questions could distinguish men at very high risk for whom PrEP is an obvious choice.  Adherence in this group tends to be good and the benefits far outweigh the costs, both financial and other.

In other populations, the equation is not so straightforward.  People at lower risk of HIV may still choose to take PrEP (or use other prevention technologies in the future) but the financial costs of preventing new HIV infections will always be higher for people who adhere less and are at lower risk.  Two papers this month consider aspects of this question.  Haberer et al. considered the overlap between PrEP adherence and risky periods within the Partners Demonstration Project, in Kenya and Uganda.  In this project, serodiscordant couples were recruited and offered PrEP if they met criteria that showed that the seronegative partner had a risk of seroconversion modelled at 3-4% per year.  Thus the seronegative population as a whole was at substantial risk.  The authors then further classified those periods where the HIV-positive participant had not yet had six months of ART and the couple had not used condoms all the time as high risk.  Prevention-effective adherence was defined as taking sufficient PrEP tablets to be effective during the periods when sex could be considered high risk.  The authors found that, reassuringly, during 75% of the time periods in their study, participants should have been protected.  This helps to explain the overall high effectiveness observed in the study and suggests that in this context people make rational decisions about when to adhere to their PrEP and when they do not need to worry so much.


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