Background: Accurate estimates of the size of the HIV-infected population, including those not yet diagnosed, are important to understand the HIV epidemic and to plan interventions. We sought to estimate the number living with HIV as well as trends in the undiagnosed population, HIV incidence, and rate of diagnosis in the past 10 years.
Methods: We used a multi-state back-calculation model to estimate HIV incidence, time between infection and diagnosis, and the HIV-infected population by CD4 count strata. The model was fitted to national surveillance data on new HIV and AIDS diagnoses from the ATHENA observational HIV cohort in the Netherlands. Rates of progression between the different states (primary infection, CD4 cell count ≥500, 350-499, 200-349, or <200 cells/mm3, and AIDS) were based on historical cohort data on untreated HIV-infected patients. Bootstrap techniques were used to calculate 95% confidence intervals (CI).
Results: By the end of 2013, 29200 (95% CI 28000-30400) individuals, of whom 23400 (22200-24600) were still alive, were estimated to have been infected with HIV since the start of the epidemic in the 1980s. Based on registered HIV cases in ATHENA, we estimated that 91% of these patients, approximately 21300 (20200-22400), were still living in the Netherlands; the remaining 9% were not in care anymore because they moved abroad or were lost to follow-up. According to the model, the number of undiagnosed HIV-infected individuals decreased from 5150 (4850-5500) in 2003 to 3400 (2500-4650) in 2013. Of the undiagnosed individuals in 2013, 23% (19-27) were estimated to have been infected for less than one year, 53% (49-56) for one to 5 years, and 24% (19-30) for more than 5 years; 53% (51-55) had CD4 counts <500 cells/mm3. The annual number of new infections remained almost unchanged: 1060 (940-1200) in 2003, 1020 (890-1151) in 2008, and 860 (590-1260) in 2013. At the time of diagnosis, the estimated proportion of patients infected less than 2 years before their HIV diagnosis increased from 21% (18-23) in 2003 to 26% (22-31) in 2013, while the proportion infected less than 5 years before increased from 60% (57-63) to 67% (62-73).
Conclusions: The number of undiagnosed HIV infections in the Netherlands is decreasing, but still almost a quarter has been infected for more than five years. Much greater increases in diagnosis rates are likely to be needed for a more substantial decrease in the annual number of new infections.