Modelling the Relationship between Antiretroviral Treatment and HIV Prevention: The Limits of Spectrum’s AIDS Impact Model in a Changing Policy Environment

This paper shows how two different publicly available epidemiological modelling packages (namely the Spectrum AIDS Impact Model and the ASSA 2003 model) predict very different impacts of rolling out highly active antiretroviral treatment (HAART) on new HIV infections. Using South Africa as a case study, it shows that ASSA 2003 predicts a significant drop in new HIV infections as HAART is rolled out, whereas Spectrum assumes that HAART will not have a preventative impact (and in fact generates a small increase in new HIV infections). Users will thus draw different conclusions about the public health benefits of HAART depending on which modelling package they use. Despite being presented as a policyoriented modelling tool capable of exploring ‘what if’ questions about the impact of different policy choices, the Spectrum model is ill-equipped to do so with regard to a HAART rollout. Unlike Spectrum, the ASSA 2003 model is more flexible and transparent. Better modelling and more information (including about the relationship between HAART and risky sex) is required to develop appropriate public policy modelling for the HAART era.