A PrEP programme targeting 25% of the highest-risk gay and other men who have sex with men in New York City could potentially prevent between 4% and 23% of the new infections predicted to occur in the next five years, according to a modelling study reported in the September 12th issue of AIDS. Over half of these infections would be prevented in men who were not themselves taking PrEP, but due to reduced overall HIV prevalence. The cost was estimated at US $31,970 per quality-adjusted year of life saved.
Pre-exposure prophylaxis (PrEP) is a potential prevention method in which antiretroviral drugs are taken before a possible HIV exposure, to reduce the risk of infection. One approach being considered is the ongoing, daily use of such prophylaxis for people with frequent high-risk exposures.
Until clinical studies (currently underway) provide the answers, questions remain as to how effective – and how cost-effective – PrEP programmes might prove. One mathematical modelling study presented at the 16th International AIDS Conference found that PrEP would be cost-effective as long as it proved more than 50% effective.
In this study, another group of researchers used a mathematical model to simulate the possible effects of a five-year PrEP distribution programme among gay and other men who have sex with men (MSM) in New York City whose sexual activity puts them at risk of HIV. The objectives were to predict the number of HIV infections that could be prevented, and the cost-effectiveness of the programme compared with current prevention practices.
Key assumptions
The model was based on published epidemiologic data for New York City, and assumed a PrEP programme targeted to the "highest-risk" MSM – 30% of the total MSM population. The price was fixed at US $31 daily – the average US wholesale price of FTC/tenofovir (Truvada, the drugs currently considered the best candidates for PrEP). To allow for uncertainties in estimates, several key parameters were each assigned a range of different values: rate of adherence, 33%, 50%, or 95%; percentage of high-risk MSM reached by the programme (population coverage), 2.5% (1500 high-risk MSM covered) or 25% (15000 covered); and PrEP efficacy, 0%, 30%, 50% or 70%. The way in which efficacy related to other parameters (adherence and levels of viral exposure) was also calculated in several ways.
Working out of these different possible estimates resulted in a total of 36 hypothetical scenarios. Furthermore, within each of these 36 scenarios, epidemiological factors (including numbers of new sexual partners and probability of HIV transmission) were varied 200 times. All in all, a total of 7200 simulations were run, each with a different configuration of parameters.
Results
Based on current incidence rates, 19,510 new HIV infections are expected to occur among all MSM in New York City over the course of the next five years,
By making PrEP available to 25% of the highest-risk MSM in the city, the simulations predicted that between 4% (780) and 23% (4510) of these infections could be prevented. More than half of these prevented infections would be among those not accessing PrEP directly themselves, but would be due to reduced HIV prevalence in the community due to the PrEP programme.
The "base-case" scenario made the following assumptions: 25% of high-risk MSM (15,000) were reached; 50% of the men in the programme were fully adherent; PrEP was 50% effective at 100% adherence and 0% effective otherwise. Under these assumptions, 1710 new HIV infections (8.7% of those expected) would be averted (90% confidence interval [CI], 306 to 2947). Seven hundred of these averted infections were directly due to PrEP use, and 1010 secondary cases due to reduced prevalence. The incremental cost was US$ 31,970 per quality-adjusted years of life [QALY] saved.
Varying the parameter models resulted in a worst-case scenario of zero benefit (no new infections averted), and a best case of 23%, or 4510 (90% CI, 3144 to 6129). Across all assumptions, PrEP was cost-effective 75% of the time, using a threshold of US $50,000 per QALY saved.
Coverage rate appeared to be the most critical variable. Estimated reductions of 4% to 23% were seen at coverage rates of 25% of high-risk MSM; when only 2.5% were covered, not enough new infections were averted to justify the intervention.
The report concluded that, although subject to many still largely unknown variables, a programme of "HIV chemoprophylaxis [PrEP] among high-risk men who have sex with men in a major US city could prevent a significant number of HIV infections and be cost effective… over a broad range of … variables." They believe that these findings "should give strong impetus to the ongoing chemoprophylaxis trials as well as to research on potential program implementation."
Reference:
Desai K et al. Modeling the impact of HIV chemoprophylaxis strategies among men who have sex with men in the United States: HIV infections prevented and cost-effectiveness. AIDS 2008;22:1829–1839.