I'd like to applaud the authors for an innovative and clinically-relevant analysis - I think the risk calculators presented here could be very useful, and I hope they gain wide use.

I have some questions and concerns with regard to the cost-effectiveness analysis, however. In the methods section, the authors note that "we modeled a 1-year duration of PrEP intervention costs and effectiveness to better reflect real-world patient-provider treatment plans." From looking at their model calculations, I believe this means they calculated, over a 1-year horizon: 1) the costs of providing PrEP, and 2) the number of HIV infections averted by PrEP.

However, in their cost-effectiveness analysis, it appears to me that the authors then multiplied their 1-year estimate of infections averted by lifetime estimates of: 1) the costs averted by preventing one case of HIV, and 2) the QALYs gained by preventing one case. In essence, this approach seems to assume that if using PrEP can avert an HIV infection for 1 year, it will then confer perfect lifelong protection from HIV for that person - at zero cost! Needless to say, this would be a very tenuous assumption. And this assumption could result in a substantial underestimation of the cost-effectiveness ratio of PrEP.

Could the authors confirm if my interpretation of their methods is correct? Please correct me if not!

Eric Ross, University of Michigan Medical School