Background: By fitting extensive national surveillance data to a mathematical model we previously reported a resurging HIV-1 epidemic amongst MSM in the Netherlands between 2002 and 2004 after a period where we found evidence for a retracting epidemic after the introduction of cART in 1996. It appeared that the epidemiological benefits of cART and diagnosis earlier in the course of infection on the HIV incidence was entirely offset by an increase in risk behaviour. Here we present an extended analysis including new data obtained in 2004-2006, validating our model outcome against annual changes in CD4 count at diagnosis over the time period 1980 - 2006.

Methods: Using our mathematical model framework we estimate average changes in unsafe sex and time from infection to diagnosis by fitting to annual HIV-1 and AIDS diagnosis data, and calculate R(t), the average number of new infections generated by each infected person. R(t) > 1, indicates an increasing epidemic.

Results: Between 2004-2006, R(t) appeared to be 1.00 (95% CI 0.91 – 1.05), indicating a stable growing HIV-1 epidemic among men having sex with men, despite the further decrease in time to diagnosis to 2.47 years (2.21 – 2.73). A qualitative comparison of CD4 cell counts at diagnosis obtained from the model versus the data showed similar temporal trends. The recent increase in the proportion of newly diagnosed individuals with high CD4 cell counts corroborates our model inferences that the recent increase in annual number of new HIV diagnoses reflects a recent rise in HIV transmission. We estimated that 20% of HIV positives were unaware of their infection at the beginning of 2007 taking account for 89% of the new infections.

Conclusions: Sexual risk-behaviour of MSM unaware of their infection drives the continuous epidemic spread of HIV-1 in The Netherlands. Our model has shown to be a robust and valuable tool for studying HIV epidemics in times of combination anti-retroviral treatment.