National Health Insurance
Modelling of the Epidemic
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Figure 1 above is taken from the latest version of a series of models produced under the guidance of Prof Rob Dorrington and the Centre for Actuarial Research (CaRE) at UCT, together with the AIDS Committee of the Actuarial Society of South Africa (ASSA). These models have been critical in developing estimates of the future costs of the epidemic in South Africa and the need for treatment. Tracing the development of the models gives insights into how these models are now structured.
Dorrington describes the early model developed by Peter Doyle of Metropolitan. “This program modelled a hypothetical population by essentially dividing the population into four groups depending on the ease with which individuals were expected to contract and transmit the HIV.” The model was proprietary and the ASSA AIDS Committee preferred to have a program which could be altered, leading to the first in a series of models, ASSA500.
A concern at the time was that “by and large demographers in this country were ignoring the impact of AIDS in their models”. This “led to the conclusion that it might be useful to produce a combined AIDS and demographic model for the country as a whole.” The resulting ASSA600 model was calibrated to replicate “as far as possible the empirical observations. Such an exercise is by its nature perhaps inevitably a little more art than science but briefly the aim was to set, where possible, the assumptions to be consistent with empirical studies and where this was not possible to set the assumptions by trial-and-error (within bounds of reasonableness) so that the output from the model reproduced the observations of the epidemic.” This and subsequent models have been calibrated to emerging population, mortality and fertility data, the annual antenatal clinic surveys of the Department of Health and to other surveys and research.
The ASSA2000 model was a model of the impact of HIV/AIDS on the South African population by population group at provincial level. Dorrington found that “modelling the epidemic at the level of the population group and provinces improves our understanding of the dynamics of the epidemic as well as our estimates of its future trajectory”. The model was released in June 2001 and subsequently modified. It was later superseded and users are cautioned by ASSA “that the ASSA2000 version is now considered to have overstated the extent of the South African epidemic. This is partly because the ... model does not allow for the effects of HIV prevention and treatment programmes ...” .
A derivative model, the ASSA2000 Orphan Model, allowed estimates of the future number of maternal, paternal and “double orphans” to be made. A useful technical note by Leigh Johnson provides an introduction to the mathematics used in these models. At that time, the issue of orphanhood in the epidemic had barely received attention and the models were instrumental in highlighting the magnitude of the problem. The authors said “South Africa can expect to see an alarming growth in the number of orphaned children over the next 15 years. ...South Africa’s capacity to provide care for these orphaned children will ... determine the long-term social stability of the country”.
Other variations on the basic model were developed. A significant development was the ASSA2000 Interventions Model which pioneered the modelling of interventions, including antiretroviral treatment. The underlying ASSA2000 model was then substantially revised to include the very useful prevention and treatment approaches in the main model. The ASSA2002 model, released in July 2004, was replaced with ASSA2003 in November 2005 and this is the current model in use. These models have a similar architecture and continue to be calibrated against emerging data on the population and the epidemic.
The ASSA2003 model allows behavioural change and changes in HIV transmission as a result of the following interventions:
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improved treatment for sexually transmitted diseases (STDs);
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information and education campaigns (IEC) and social marketing;
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voluntary counselling and testing (VCT);
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mother-to-child transmission prevention (MTCTP); and
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antiretroviral treatment (ART).
Significantly, the model allows for determining the numbers at various stages of HIV infection: “In the absence of antiretroviral treatment, adults are assumed to progress through four stages of disease before dying from AIDS. These four stages correspond to those defined in the WHO Clinical Staging System. The effects of antiretroviral treatment (ART) are modelled by introducing a further two stages, which represent people receiving treatment and people who have started treatment but subsequently discontinued treatment.” This is illustrated below.
 Figure 2: Staging of HIV Infection in South Africa from 1985 to 2025, using the ASSA2003 Model with standard assumptions about treatment and interventions
The rationale for the total number of people with HIV/AIDS levelling off and remaining at about the same level can be seen by referring back to Figure 1. The number of new infections declines and the number of deaths increases until the two are almost in balance.
The ASSA2003 suite of models includes several versions:
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South Africa ‘lite’ version: treats the population of the country as one population group;
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South Africa ‘full’ version: separate modelling of each of four population groups (Asian/Indian, black African, Coloured and White);
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Provincial version: the application of the full version of the model separately to each of the nine provinces.
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Urban-rural version: this allows for situations where there is significant migration between two groups with significantly different prevalence levels.
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The Orphans model remains an additional module that can be run with any of the above.
The ASSA2003 and related models have been calibrated by students for some other African countries. The ASSA Select Model needs to be used when considering the impact on a defined group drawn from the large population, like an industry or the workforce in a particular company.
Dr Leigh Johnson of UCT completed his PhD thesis in 2008, capping a significant contribution to the modelling of the epidemic with a model on the interaction between HIV and other sexually transmitted infections in South Africa. The impact of the models developed by Prof Rob Dorrington and Dr Leigh Johnson is that we have available excellent estimates of the population and the course of the epidemic for the work to come on costing National Health Insurance.
It was argued in IMSA NHI Policy Brief 1 that all costing work on National Health Insurance should be done using the ASSA2003 provincial model and that the costings be updated when a revision to the model is released. An update is under consideration and the aim is to release it by the end of 2009.
The ASSA2003 provincial tables by age and gender from 1985 to 2025, including a summarised and a detailed staging of HIV infection, can be downloaded as a spreadsheet from the IMSA web-site.
Personal communication, Dr Leigh Johnson, 2 July 2009.
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