Behavioural risk variables in the models consisted of baseline behaviour measures and behaviour change measures. The first varied by location but were well evidenced. For the behaviour change set of variables we found, firstly, that the direction of the estimate varied (risk increase, decrease, or no change); and, secondly, that the use of the evidence-base for the change scenarios also varied depending on whether increased or decreased risk scenarios were assumed. We describe these findings in turn.
Uses of evidence
Variations in assumptions of behaviour change and their inclusion in studies were linked to different uses of evidence.
In studies where it was acknowledged that assumptions about PrEP-related behaviour change were not included, authors mentioned other several possible clinical, behavioural and social variables also not included in the analysis. For example, Kessler et al int heir study explain:
"There are critical elements relating to the norms of people of various sexual identities and behavioural patterns, including differential condom use with casual partners, serosortive and seroadaptive practices, and alterations in risk behaviour as a result of HIV awareness, that are not explicitly accounted for in our computer simulation due to inherent choices and trade-offs made between model complexity and transparency” (25).
These authors also acknowledged that, “Many of these elements may act to reduce individual level risk for HIV acquisition” (Ibid), thus it would be challenging to isolate the effect of PrEP use itself, given such a range of possible variables. Furthermore, it would be difficult to determine if PrEP use influenced any related element of risk reduction, so assuming PrEP had such effects may even “overestimate the actual health benefits of PrEP" (Ibid).
In a second article that acknowledged not including assumptions about PrEP-related behaviour change, authors explained the “analysis does not explicitly account for many complexities of HIV transmission and treatment, including effects of PrEP use on treatment resistance, correlations between sexual risk behaviour and treatment outcomes" (26).
In studies with assumptions of increased risk behaviour associated with PrEP, authors related to available evidence in three ways. Firstly, they inferred quantitative estimates from qualitative studies about perceptions of likely behaviours among potential PrEP users, some of whom reported behaviour change was possible. For example, a study about the cost-effectiveness of PrEP in the US (27) based assumptions of increased risk behaviours associated with PrEP on three qualitative studies conducted before PrEP was approved by the FDA: one conducted from 2007 to 2009 (28); a second conducted in 2009 (29); and a third completed in 2011 (30). These studies solicited preferences and perceptions among men who have sex with men in New York, couples in San Francisco, and African-American men in Atlanta. In these studies, investigators found 10% of men were concerned that PrEP could lead to behaviour change (30), however “participants differed about whether risk-reduction behaviours would change, and in which direction” following the introduction of PrEP (29). Based on these findings, the authors of the cost-effectiveness study represented behavioural disinhibition in three ways, including: “15% decrease in condom use, 15% increase in sexual encounters, and resulting 15% increase in STI prevalence among those taking PrEP” (27).
Secondly, authors grouped assumptions about behaviour compensation with estimates of lower adherence. As a result, behavioural disinhibition was unduly associated with reductions in clinical efficacy. For example, the authors suggested, “some individuals on PrEP may engage in behavioural disinhibition (be willing to adopt riskier sexual behaviour as a result taking PrEP), and adherence to a daily preventive regimen may vary widely, leading to different levels of clinical effectiveness"(27). The authors associated both “riskier sexual behaviour” and variable adherence with decreases in clinical efficacy: "PrEP efficacy decreased from 44–28%, thus increasing the number needed to treat to 97 (95% UR: 46 to 222)." (27): Page 4).
Thirdly, studies projecting increased risk associated with PrEP use cited evidence of increased STI diagnoses. Two studies presented a conflicting picture with one reporting that whilst STI rates were high, the increase was not necessarily linked with PrEP use, with the other concluding that increased STI incidence was attributable to risk compensation when taking PrEP: ‘30% of patients were diagnosed with a STI after 6 months and 50% after 12 months on PrEP…..there were no incidences of accompanying HIV infection’ (31). Another used the increase in STIs to justify estimates of increased HIV incidence over the lifetime of PrEP users, whether or not they continued to use PrEP. For example, one study assumed “Risk compensation would also lead to an increase in HIV exposure” and resulting 20% increase in HIV incidence (32). To justify this assumption, the authors wrote, “published evidence suggests increased frequency of anal CLS subsequent to PrEP use and increased STI diagnoses” (32, 33). While the evidence cited in this instance does show moderate increases in STIs over time, the same cited evidence also clearly observes, “no HIV seroconversions occurred during PrEP use" (23), and does not provide data to suggest that PrEP-related risk compensation will lead to 20% increase in HIV incidence over lifetime of those given PrEP.
In studies with scenarios of decreased risk behaviour associated with PrEP, authors leveraged evidence in two key ways. In the first instance, the authors referred to previous research about risk reduction associated with related elements of HIV care and prevention, including counselling associated with HIV testing, knowledge of one’s own HIV status and treatment as prevention (TASP) (34–36). For example, one study citing this evidence observed, “Providing counselling with HIV testing has been found to reduce risky sexual behaviour; thus, we assumed a 20% reduction in risky behaviour for both infected and uninfected persons after HIV screening” (37). In the second instance, studies cited literature that argues for the importance of reducing risk behaviour in order to improve the cost-effectiveness of HIV care, not empirical studies that demonstrate reduced risk behaviour. In one such study citing this literature, authors also assumed, “20% reduction in risky sexual behaviour owing to testing and counselling associated with PrEP” (38).
The single study that assumed a wide range of possible effects on user behaviour reasoned there was the potential for behavioural disinhibition, as evidenced by research about previous risk reduction methods (39) however the authors also recognized the “issue remains to be resolved empirically for the particular case of PrEP,” thus they “considered a broad range of behavioural assumptions”(40).
Across all studies, whether assuming increased risk behaviour, decreased risk behaviour or no change in behaviour associated with PrEP use, authors made use of available evidence for different ends, referring to different resources, at times incomplete. In some cases, the particular use of evidence had significant effects on cost-effectiveness results. For example, when authors constructed a scenario that grouped behavioural disinhibition with lower adherence and thus suggested both would be associated with a reduction in the clinical effectiveness of daily PrEP, they lowered the relative risk reduction of the intervention from 44–28%, elevated the projected number needed to treat (NTT): from 64 [26-176] to 97 [46-222], and raised the cost per quality-adjusted life-years gained from $160,000 to $320,000 ((27): table II).