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closePredictions from the Abbas et al. PrEP Implementation model are incorrect due to flaws in their model structure
Posted by sblower on 26 Nov 2007 at 02:22 GMT
Simplified Flow Diagram of Model with PrEP Implementation.
http://plosone.org/article/info:doi/10.1371/journal.pone.0000875#pone-0000875-g001
The equations proposed by Abbas et al. for their model of PrEP implementation are represented by their simplified flow-diagram shown in Figure 1. Examination of their flow-diagram shows that Abbas et al. have made substantial errors in the assumptions that they made when constructing their model. Specifically, they assumed that:
(1) drug-resistant strains of HIV can evolve in individuals who are NOT infected with HIV and on PrEP. This assumption is represented by the arrow in their flow-diagram that shows that “susceptible individuals that are on PrEP” can move directly to become “HIV-infected individuals who are on PrEP and have acquired secondary resistance to PrEP”. This first assumption by Abbas et al. is incorrect.
(2) drug-resistant strains of HIV can NOT evolve in individuals who are infected with drug-sensitive strains of HIV and on PrEP. This assumption is represented by the fact that there is not an arrow that allows “individuals that are infected with drug-sensitive strains of HIV and on PrEP” to move to become “HIV-infected individuals who have acquired secondary resistance to PrEP”. This second assumption by Abbas et al. is incorrect.
These two major incorrect assumptions that Abbas et al. have made when they constructed their PrEP implementation model have ensured that their predictions of the potential epidemiological impact of PrEP are incorrect.
Professor Sally Blower
Head of Disease Modeling Group
David Geffen School of Medicine at UCLA
THE AUTHORS’ FINAL REPLY
uabbas replied to sblower on 26 Nov 2007 at 18:19 GMT
The statements in the second comment posted by Professor Blower are again incorrect First, our model does not allow the evolution of drug-resistant strains of HIV in uninfected individuals. Second, our model does allow that drug-resistant strains of HIV could evolve in individuals who are infected with drug-sensitive strains of HIV and taking PrEP.
We have detailed our model equations and assumptions in our manuscript and Appendix S1 with further clarification in our first reply. We leave interested readers to draw their own conclusions from the published work.
We do not believe this dialogue is productive for the readership, hence we will not respond to subsequent postings by Professor Blower.
Ume L. Abbas, MD
John W. Mellors, MD
University of Pittsburgh, Pittsburgh, PA, USA
Roy M. Anderson, Ph.D.
Imperial College London, London, UK
RE: THE AUTHORS’ FINAL REPLY
sblower replied to uabbas on 27 Nov 2007 at 01:55 GMT
Dear Authors,
If you do not agree with my evaluation of your PrEP implementation model then please could you take a minute to explain for the interested reader:
1) Why you DO have an arrow in your flow-diagram that shows that “susceptible individuals that are on PrEP” can move directly (and instantaneously) to become “HIV-infected individuals who are on PrEP and have acquired secondary resistance to PrEP”and
2) Why you do NOT have an arrow in your flow-diagram that shows that “individuals that are infected with drug-sensitive strains of HIV and on PrEP” can become "HIV-infected individuals who have acquired secondary resistance to PrEP”.
Thank you - if you would answer these two short simple questions it would be very productive as it would allow the interested readers to assess who is correct.
Professor Sally Blower
Head of Disease Modeling Group
David Geffen School of Medicine at UCLA