In the wake of the 2010 results of the Pre-Exposure Prophylaxis Initiative (iPrEx) study – a multi-national trial of daily oral tenofovir/emtricitabine to prevent acquisition of HIV among men who have sex with men (MSM) indicating a 44% reduction in risk – there have been attempts to model the potential health impact and cost-effectiveness of Pre-Exposure
Prophylaxis interventions among MSM, according to various scenarios of prioritization of high-risk groups (e.g. Cremin, Garnett et al.: http://sti.bmj.com/content/87/Suppl_1/A36.1.abstract?sid=a288a0b2-af68-40d5-8b61-1ea27b632395). A recent modelling study based on epidemiological data of MSM in Lima, Peru – Gomez, Hallett et al. – previewed for STI journal last year as a late breaker abstract (http://sti.bmj.com/content/87/Suppl_1/A350.2.abstract?sid=a288a0b2-af68-40d5-8b61-1ea27b632395), has now been published in PLoS Medicine. It brings out with particular clarity the potential impact on cost-effectiveness of focussing limited resources on a relatively narrow, high-risk sector of the MSM population such as trans-women and sex-workers http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fjournal.pmed.1001323.
Why is this interesting? Gomez, Hallett et al. make an apt comparison of the total cost of introducing PrEP in Peru on a sufficient scale to reduce new infections by a third – US$20 million p.a. over 10 years – and total current HIV spending p.a. in Peru (prevention, treatment and care) – US$40 million p.a.. In short, given an insufficiency of resources, it is not enough that an intervention be cost-effective, it must also be affordable. This model considers relatively modest levels of PrEP MSM coverage of 5% (low) and 20% (high), respectively – and for each of these levels of coverage looks at the impact on cost-effectiveness of different levels of prioritization. The cost of ART is not taken into account in the calculation of costs averted.
Practically all the PrEP interventions considered fall below the WHO threshold of cost-effectiveness (3 times per cap. GDP per DALY averted), but a low coverage and high prioritization intervention would meet even the more stringent World Bank cost-effectiveness threshold of <$500 per DALY averted. Such an intervention has a cost-effectiveness that is comparable with the cost-effectiveness of STI treatment interventions, even if it is less cost-effective that voluntary counselling and testing and condom provision (Aldridge, Miranda et al.: http://researchonline.lshtm.ac.uk/4538/1/1471-2458-9-352.pdf).
Such considerations may one day prove very useful to governments seeking to make policy decisions on resource allocation that are in line with UNAIDS guidelines, which stress both the importance of cost-effectiveness and the potential of prioritizing high-risk groups (http://www.unaids.org/en/media/unaids/contentassets/documents/unaidspublication/2011/JC2216_WorldAIDSday_report_2011_en.pdf).
The general message of the paper seems to be that PrEP is no panacea, but could in future make a contribution as part of a combination package even in limited resource settings, especially if policy makers consider the option of a low coverage narrowly focussed on high-risk groups. Yet, for the present, the feasibility of PrEP as a future public health intervention seems fraught with uncertainty. Firstly its effectiveness is hard to determine where levels of adherence are also uncertain (http://sti.bmj.com/content/87/Suppl_1/A1.4.abstract?sid=2ffb6e56-e489-49f5-8ea0-eb291723644f). Secondly, there is the impact of risk compensation behaviour (http://sti.bmj.com/content/88/4/258.abstract?sid=2ffb6e56-e489-49f5-8ea0-eb291723644f), and of pill sharing (http://sti.bmj.com/content/87/Suppl_1/A35.2.abstract?sid=2ffb6e56-e489-49f5-8ea0-eb291723644f). Both of these would be likely to ensue with the attendant risks of heightened drug resistance, where PrEP is deployed in limited-resource settings.