Assessment of epidemic projections using recent HIV survey data in South Africa: a validation analysis of ten mathematical models of HIV epidemiology in the antiretroviral therapy era

October 1, 2015


Background: Mathematical models are widely used to simulate the effects of interventions to control HIV and to project future epidemiological trends and resource needs. We aimed to validate past model projections against data from a large household survey done in South Africa in 2012.

Methods: We compared ten model projections of HIV prevalence, HIV incidence, and antiretroviral therapy (ART) coverage for South Africa with estimates from national household survey data from 2012. Model projections for 2012 were made before the publication of the 2012 household survey. We compared adult (age 15–49 years) HIV prevalence in 2012, the change in prevalence between 2008 and 2012, and prevalence, incidence, and ART coverage by sex and by age groups between model projections and the 2012 household survey.

Results: All models projected lower prevalence estimates for 2012 than the survey estimate (18·8%), with eight models' central projections being below the survey 95% CI (17·5–20·3). Eight models projected that HIV prevalence would remain unchanged (n=5) or decline (n=3) between 2008 and 2012, whereas prevalence estimates from the household surveys increased from 16·9% in 2008 to 18·8% in 2012 (difference 1·9, 95% CI −0·1 to 3·9). Model projections accurately predicted the 1·6 percentage point prevalence decline (95% CI −0·3 to 3·5) in young adults aged 15–24 years, and the 2·2 percentage point (0·5 to 3·9) increase in those aged 50 years and older. Models accurately represented the number of adults on ART in 2012; six of ten models were within the survey 95% CI of 1·54–2·12 million. However, the differential ART coverage between women and men was not fully captured; all model projections of the sex ratio of women to men on ART were lower than the survey estimate of 2·22 (95% CI 1·73–2·71).

Five models projected that prevalence in adults aged 15–49 years in 2012 would change by 0·3 percentage points or less from prevalence in 2008. Three models projected prevalence declines of 0·7 to 1·3 percentage points. (figure 1). One model, STI-HIV, projected an increase in prevalence of 0·9 percentage points (95% CI 0·6–1·4; figure 1C).

Figure 1: Projections for HIV prevalence in adults aged 15–49 years

(A) Projected trend for prevalence in adults aged 15–49 years from 2002 to 2012. Circles show national survey estimates in 2002, 2005, 2008, and 2012, with 95% CIs (bars). (B) Model projections for prevalence in adults aged 15–49 years in 2012 and (C) absolute (percentage point) change in HIV prevalence in adults aged 15–49 years from 2008 to 2012. Dashed horizontal lines show household survey estimates and grey regions show 95% CIs around these estimates. Shaded areas around model projections show 95% Bayesian credible regions for the three models that estimated statistical uncertainty around model estimates and projections. EPP=Estimation and Projection Package.


Interpretation: Projections for overall declines in HIV epidemics during the ART era might have been optimistic. Future treatment and HIV prevention needs might be greater than previously forecasted. Additional data about service provision for HIV care could help inform more accurate projections.