Modelling ART impact on HIV prevention within discordant couples

The HPTN 052 study, discussed in an earlier blog, appears to offer a godsend:  a demonstrated 96% reduction in HIV transmission in discordant couples using antiretroviral therapy (ART).   Apparently, the extension of ART conveys an additional, and unexpected, benefit in its potential impact on prevention.  But how do we quantify the additional benefit?

A recent concise communication in the journal AIDS claims that the benefit depends very much on the country you live in.  They claim that mathematical modelling can help us pick out the countries where the benefit is greatest – and consequently most likely to outweigh the various costs and dis-benefits of ART extension.  Feed into their model the three factors of population size, HIV prevalence and the percentage of discordant couples, and it will throw up results that common sense would not have predicted – as, for example, that Malawi, with its potential combination of a reduction in HIV incidence and large number of prevented infections, would benefit more than Ghana, Rwanda or Lesotho (the other countries featuring in the study).

The most striking finding of this modelling exercise is the importance of the stability of discordant couples. When the model estimates stability at 40% or 70% respectively there is a dramatic change in effectiveness of ART extension as a prevention strategy.    Even at the higher estimate, however, the prevention of HIV within discordant couples does not result in noticeable decline in national incidence overall, where prevalence is at the levels seen in Rwanda and Ghana.  This only occurs where incidence reaches higher levels seen in Malawi and Lesotho. Evidently the extension of ART, even to a large proportion of discordant couples, offers no panacea.

It is possible that a modelling exercise of this kind might enable policy makers to pick out the countries where the benefits of ART extension would be greatest.  But so far as the countries considered by the paper are concerned (Malawi, Lesotho, Rwanda, Ghana), the presentation of data in this paper is blatantly misleading.  Why do the authors give equal weight, for each country, to the absolute number of infections prevented alongside decline in incidence?  What purpose is served by treating the former as an independent factor in determining the benefit derived by each country, when there are such differences in the size of their respective populations?  We soon discover.  The authors’ conclusion that Malawi would derive greater benefit than Lesotho appears to be entirely based on the fact that in the former case the decline in incidence will be accompanied by a much higher number of infections prevented.  This leaves out of account the relative size of the two populations – and the population of Malawi is almost ten times that of Lesotho!

Have the authors rushed this paper out in response to the results from  HPTN 052?

On HPTN 052:

http://www.hptn.org/web%20documents/PressReleases/HPTN052PressReleaseFINAL5_12_118am.pdf

Wafaa M. El-Sadr, Brian J. Coburn and Sally M. Blower, “Modeling the impact on the HIV epidemic of treating discordant couples with antiretrovirals to prevent transmission”:, AIDS Volume 25, issue 18, 28th November 2011

http://journals.lww.com/aidsonline/Abstract/2011/11280/Modeling_the_impact_on_the_HIV_epidemic_of.12.aspx

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