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Given the extent of data on the natural history of HIV infection and the effect of antiretroviral therapy (ART), it should be possible to develop a model that encapsulates and can simulate these processes, providing a means of exploring various clinical and epidemiological questions. We aimed to develop such a model and use it to reconstruct the HIV-infected population in the UK to 2006.A stochastic computer simulation model was developed that incorporates much of our understanding of the underlying processes of HIV disease progression and the effect of ART.The model generally fitted well to a range of data in treated and untreated infection. UK reconstructions suggest that, of around 68 500 people alive with HIV infection at the end of 2006, around 34 000 (49%) were on ART, with an increasing proportion of these on first-line regimens (75% in 2006). The number of patients who have failed virologically on the original three main drug classes (estimated at around 4300 in 2006) is increasing only gradually, and an increasing proportion of these patients have suppressed viral load.The beneficial effects of ART at a population level look set to continue as the number of patients exhausting the three original drug classes remains small.