Targeting Truvada (tenofovir/emtricitabine) as pre-exposure prophylaxis (PrEP) to New York City men who have sex with men (MSM) at high risk for HIV is the most cost-effective approach to promoting the intervention. However, this strategy’s price tag would approach half a billion dollars a year; and the likelihood that it would save money in the long run is relatively slim. Publishing their findings in the journal AIDS, researchers created mathematical models to compare projected outcomes of targeting PrEP to different risk groups in New York.

The base case scenarios were a 20-year period without PrEP, in which there would be 58,024 new cases of HIV in New York, and such a period in which PrEP was prioritized to all at-risk groups, including MSM, high-risk heterosexuals and injection drug users (IDUs). These scenarios were then compared with promoting PrEP to all MSM, to only high-risk MSM, to high-risk heterosexuals, and to IDUs, as well as to every combination of these four risk groups.

The researchers based their calculations on a 44 percent effectiveness of PrEP across the population receiving prescriptions for Truvada, on the notion that 50 percent of the population to whom the drug was targeted would agree to go on PrEP, and on an annual cost per person of $9,762.

The iPrEx study that first proved PrEP’s efficacy among MSM in 2010 found that, among the entire group that was assigned to take Truvada, there was a 44 percent reduction in new HIV cases when compared with the group that received a placebo. This is called the “intent to treat” efficacy, which can be characterized as a population-level efficacy. This figure takes into account an average of how well everyone adhered to the drug, since adherence is tied to the drug’s efficacy in individuals. The iPrEx open-label extension (OLE) trial, published in July, showed about a 50 percent population-level efficacy.

However, researchers analyzing the results of iPrEx and iPrEx OLE estimated that those who took Truvada every day were 99 percent protected according to iPrEx and 100 percent protected according to iPrEx OLE. No one in either study contracted HIV if they took Truvada four or more days a week. The reason that there were nevertheless two different estimates of the efficacy of taking PrEP daily is because researchers used different types of math in order to make the respective estimates.

So while the mathematical model in this new study assumes that half of a given targeted population will agree to take Truvada, it also assumes that, as in the clinical trials of PrEP, adherence to the drug will be relatively poor, thus dragging down the population-level efficacy.

Prioritizing PrEP for all groups would avert 24 percent of new HIV infections, according to the model, for a cost of $49.6 million per infection averted. Prioritizing PrEP for all MSM would result in a projected 19 percent reduction in new HIV infections in New York City when compared with an absence of PrEP. When compared with a strategy of promoting PrEP to all at-risk individuals, narrowing the field just to MSM would cost just 15 percent of the other option while retaining 79 percent of the reduction in new HIV cases. By prioritizing just high-risk MSM, 60 percent of the reduction in HIV would be retained at just 6 percent of the total cost. This latter strategy could be categorized in the general realm of cost effectiveness, but it would not save money. The annual cost for this strategy would be $467 million annually, for a cost-per-infection averted of $1.1 million.

However, if PrEP were prioritized to all high-risk MSM and 70 to 100 percent of them agreed to go on Truvada, and if the cost of the drug were cut in half, this strategy would save money (by costing less than $360,000 per infection averted) as it would prevent as much as 40 percent of new infections.

To read the study abstract, click here.