Modelling national HIV/AIDS epidemics: revised approach in the UNAIDS Estimation and Projection Package 2011

Abstract

Objective: United Nations Programme on HIV/AIDS reports regularly on estimated levels and trends in HIV/AIDS epidemics, which are evaluated using an epidemiological model within the Estimation and Projection Package (EPP). The relatively simple four-parameter model of HIV incidence used in EPP through the previous round of estimates has encountered challenges when attempting to fit certain data series on prevalence over time, particularly in settings with long running epidemics where prevalence has increased recently. To address this, the most recent version of the modelling package (EPP 2011) includes a more flexible epidemiological model that allows HIV infection risk to vary over time. This paper describes the technical details of this flexible approach to modelling HIV transmission dynamics within EPP 2011.

Methodology: For the flexible modelling approach, the force of infection parameter, r, is allowed to vary over time through a random walk formulation, and an informative prior distribution is used to improve short-term projections beyond the last year of data. Model parameters are estimated using a Bayesian estimation approach in which models are fit to HIV seroprevalence data from surveillance sites.

Results: This flexible model can yield better estimates of HIV prevalence over time in situations where the classic EPP model has difficulties, such as in Uganda, where prevalence is no longer falling. Based on formal out-of-sample projection tests, the flexible modelling approach also improves predictions and CIs for extrapolations beyond the last observed data point.

Conclusions: We recommend use of a flexible modelling approach where data are sufficient (eg, where at least 5 years of observations are available), and particularly where an epidemic is beyond its peak.

Figure 1

Estimation and projection of HIV prevalence and incidence in 10 countries with generalised epidemics: Botswana, Ethiopia, Gabon, Ghana, Kenya, Namibia, Rwanda, Tanzania, Uganda, and Zambia. Coloured dots show observed prevalence from different sites. The black solid line is the median of the classic Estimation and Projection Package (EPP) model trajectories, the blue solid line is the median of the flexible model trajectories, the dashed lines are the 95% credible intervals, and the red solid line is the data trend averaged over all clinics at each year.

Figure 2

Estimation and projection of HIV prevalence among high risk populations in Mexico, Ukraine, Moldova, Nepal, Australia, and Algeria. The high risk populations are sorted by columns: injecting drug users (IDU), men who have sex with men (MSM), female commercial sex workers (FSW), and clients of sex workers (Client). Coloured dots are observed prevalence from different sites. The blue solid line is the median of the flexible model trajectories and the blue dashed lines show the 95% credible intervals of the flexible model trajectories.

Figure 3

Out-of-sample projection of HIV prevalence and incidence in 10 countries with generalised epidemics: Botswana, Ethiopia, Gabon, Ghana, Kenya, Namibia, Rwanda, Tanzania, Uganda, and Zambia. Coloured dots show observed prevalence from different sites. The black solid line is the median of the classic Estimation and Projection Package (EPP) model trajectories sampled from the posterior distribution, the blue solid line is the median of the flexible model trajectories, the dashed lines are the 95% credible intervals, and the red solid line is the data trend averaged over all clinics at each year. The data to the right of the vertical line are used only to validate the projection but not for estimating the model parameters.