Modeling the potential effectiveness of PrEP as against other preventative interventions in addressing MSM HIV

 

Despite the known preventative benefits of ART, the incidence of HIV among UK MSM population has remained relatively constant over the last 10 years and looks set to remain so. The UN 90:90:90 target will soon be achieved for this population, yet the goal of eliminating the infection seems no nearer.  Not surprisingly there is an appetite among health professionals for alternative measures – like PrEP.

A recent study (Punyacharoensin & White) claims to be the first to model, on the basis of detailed behavioural and surveillance data, the  impact of seven potential interventions – including PrEP – on HIV incidence in this population.  There is a considerable diversity in the nature of these interventions.  One of them consists in the roll-out of the recent WHO policy of ‘diagnose and treat’: others in the achievement of specific targets, such as yearly HIV testing for a given percentage of MSM: others again in the fulfilment of what seem little more than aspirations, given the absence of any indication of how they might be achieved – such as halving one-time partners or unprotected anal intercourse. The potential impact of each intervention is assessed independently, and then in various combinations with other interventions.

It is evident from the way that the various combinations of interventions are grouped into two sequences that the primary issue for these authors is the potential of PrEP (which happens to be the only genuinely new tool in the box).  The first sequence combines PrEP at varying levels of coverage and effectiveness with the achievement of one-year testing for different proportions of the population, ‘test and treat’, and various levels of risk compensation.  The second sequence goes through much the same process but with PrEP now replaced by a putative 0.5 drop in repeat partnerships.  The main study outcome appears to be the demonstration that introducing PrEP at certain levels of coverage and effectiveness could, in certain combinations, have an impact on incidence (-43.6%) that would be of the same order (-41%) as the achievement (by unspecified means) of a putative 0.5 drop in repeat partnerships.

The biggest problem for the modelers, of course, is the difficulty of predicting the effectiveness of PrEP in a real-life setting.  When empirical evidence of this finally emerges, then the value of this intervention as against traditional interventions will presumably be determined by its relative cost-effectiveness.  In other words, the question to confront health policy makers will be how much of what they would otherwise have spent on traditional interventions should be diverted into PrEP.

In the meantime, if there is key message to emerge from this study, it is that PrEP is worth a try.