Frequently asked questions



How does Thembisa differ from Goals, Spectrum and EPP?


Goals, Spectrum and EPP are models that can be applied in any HIV epidemic setting. Thembisa is a model that has been developed specifically for South Africa, and no attempts have been made to apply the model to settings outside of South Africa. South Africa is a data-rich country, which means that it is possible to develop much more detailed models of the South African HIV epidemic than would be possible in many other African countries.


Goals, Spectrum and EPP are part of a suite of models, each of which serves a different function. EPP is used to obtain estimates of HIV incidence rates from HIV prevalence data, while Spectrum uses these incidence estimates to evaluate the demographic impact of HIV, and Goals is used to evaluate the impact of HIV interventions. The Thembisa model integrates these different functions into a single model.


The model-fitting procedures in EPP and Thembisa both aim to identify the model parameters that yield HIV prevalence estimates consistent with survey data. However, the Thembisa model fitting procedure works with age-specific HIV prevalence data, while the EPP model fitting procedure uses aggregated HIV prevalence estimates for the 15-49 age group. In addition, the Thembisa model is calibrated to mortality data.


The demographic components of the Spectrum and Thembisa models are similar. Both stratify the population by age and sex. Both stratify the HIV-positive population by CD4 count and antiretroviral treatment (ART) status.


The intervention components of the Goals and Thembisa model differ in several respects. Goals includes a number of sub-populations that are not currently in the Thembisa model (men who have sex with men (MSM) and people who inject drugs (PWID)), and can thus evaluate the impact of interventions specific to MSM and PWID. On the other hand, Goals is not an age-structured model, and is therefore not appropriate for considering age-specific interventions. Cross-sectional measures of coverage tend to be specified as inputs in the Goals model, whereas in the Thembisa model, coverage is usually an output and the inputs are specified either as the rate of engaging in a particular HIV service or the number of individuals engaging in a particular service for the first time. The Goals model comes with a built-in ‘impact matrix’ for determining the impact of individual interventions on key epidemiological parameters, while the Thembisa model requires that users specify the key epidemiological parameters directly, taking into account the expected impact of interventions.


How does Thembisa differ from the ASSA model?


Thembisa inherits a number of features from the Actuarial Society of South Africa (ASSA) AIDS and Demographic model, especially in the demographic structure of the model. The key differences are:


What are the limitations of Thembisa?


The Thembisa model is a model of heterosexual transmission and it does not consider MSM as a separate risk group. It also does not consider PWID as a separate risk group. This means that interventions that are specific to MSM and PWID cannot be evaluated by Thembisa in its current form.


The modelling of ART interruptions and ART discontinuation is currently quite simplistic. ART discontinuation and ART resumption are not modelled dynamically; instead the model works with an assumed fraction of ART-experienced adults who are actively on treatment at each treatment duration. Research is currently being conducted to estimate rates of ART discontinuation and resumption in South Africa, and will be used to improve the model in future.


The Thembisa model is a frequency-dependent model. This means that the rate at which an individual acquires HIV depends on the HIV prevalence in the pool of individuals with whom that individual could potentially partner. Individuals are not assigned specific sexual partners, and this means that the model cannot evaluate couple-based HIV prevention strategies (e.g. PrEP for discordant couples and couple-based HIV testing). It also means that the modelling of HIV transmission is not fully realistic.


Thembisa is a compartmental model which divides the population into discrete risk groups. In reality, sexual behaviour is too diverse to be ‘compartmentalized’. This means that the model of sexual behaviour is simplistic, and it may not adequately reflect the true level of heterogeneity in risk behaviour that exists in reality.


How should I cite the outputs of the Thembisa model?


If you are citing the most recent national statistics, please cite the following journal article:

Johnson LF, Chiu C, Myer L, Davies MA, Dorrington RE, Bekker LG, Boulle A, Meyer-Rath G. (2016) Prospects for HIV control in South Africa: a model-based analysis. Global Health Action. 9: 30314.


If you are citing the most recent provincial statistics, please cite the following report:

Johnson LF, Dorrington RE, Moolla H. (2016) Modelling the impact of HIV in South Africa's provinces. Centre for Infectious Disease Epidemiology and Research working paper. University of Cape Town. Available: