Methodological concerns regarding a PrEP model.

We would like to raise a concern regarding the mathematica l modelling study by Brooke Nichols and colleagues, which looked at the cost-effectiveness of preexposure prophylaxis (PrEP) for HIV-1 prevention in the Netherlands. Specifically, we have found that we are unable to replicate their results using the methodology described in their paper. In their modelling study the authors divided the population into four risk groups on the basis of sexual behaviour. They used a standard method, Monte Carlo filtering, for the first stage of their analysis: to calibrate their model. To do the Monte Carlo filtering, the authors assigned ranges to specify the size of the four risk groups, and then sampled each of these four ranges 100 000 times. The ranges the authors used were: 5–15% for the highest risk group, 30–50% for the second highest, 10–35% for the third highest, and 4–46% for the lowest risk group. We have rerun the authors’ analysis with the same ranges and sampled each of the four ranges 100 000 times. However, we have been unable to replicate their results.