Özge Pala Burak Güneralp Onur Mergen
BS, Boðaziçi University, Turkey BS, Boðaziçi University, Turkey BS, Boðaziçi University,Turkey
Today's present knowledge on the sexual behaviour and practices
of people, and probability of HIV transmission through various
forms of sexual intercourse is very limited. Moreover, statistical
data on demographic information of the developing countries are
both limited and unreliable. These drastically harden the modelling
studies, especially for the developing world. Still, the authors
believe that, experimenting with the simulation models of the
AIDS epidemic under the light of systems thinking, can help extend
our knowledge about this most recent plague.
The global pandemic of HIV infection is influenced by many factors - eg. time of introduction, population density, cultural and social issues. Incidence rates have been the highest in developing countries where heterosexual transmission is most common. Because of the late introduction of the epidemic, low social status of women, inadequate health infrastructure, poor economic conditions, cultural misconceptions and social inhibitions, and the density of the population, there may be more HIV infected individuals in Asia by the year 2000 than in any other region of the world. Therefore, the application area of the study was chosen to be Southeast Asia, a region which accommodates approximately 25% of the world's population and is still in the early phases of an explosive HIV/AIDS epidemic. Although HIV was introduced much later in Asia than the rest of the world, over 4 million people are now thought to be infected. WHO estimates that, with continued escalation of HIV transmission, nearly 9 million HIV-infected people will reside in South-East Asia by the turn of the century.
Major aim of this modeling study of the AIDS epidemic is to propose
certain policies and recommendations regarding the future course
of actions that will be followed by policy makers. These policies
aim to reduce the peak value of infection rate.
Sexually active population is partitioned into groups according
to their sexual preferences relying on the fact that, the infection
risk posed on the individuals is determined by the sexual activity
they are engaged in. These groups consist of heterosexual
females, commercial sex workers (CSW), male heterosexuals, male
homosexuals, and male bisexuals . The importance of distinguishing
CSW stems from the fact that commercial sex is very wide spread
For each of these groups the infection rate equation is tailored
to reflect the coupling patterns based on the sexual practices
(Pala, Güneralp & Mergen 1997). For example, the infection
rate equation for female heterosexuals describes contacts with
both male bisexuals and male heterosexuals. This rate defines
the number of HIV-negative people that become infected in a month
as a result of sexual intercourse. To calculate the total number
of people (of a specific group) that become infected in a month,
an infection probability per individual per month is computed
and then multiplied with the susceptible population of the relevant
group (Pala, Güneralp & Mergen 1997).
Public awareness is also incorporated into the model as a negative
feedback loop. The level of the surveillance of the epidemic in
the society affects the people, in terms of getting aware of the
current status of the epidemic. The increased public awareness
causes an increase in condom usage, a decrease in the number of
casual partners and a decrease in the percentage of males visiting
CSW. The changes in these factors affect the level of surveillance
of the epidemic via the infection rate, completing the loop. In
this manner, the analysis of how the awareness of the people and
the control they exert on their sexual practices affect the spread
of the epidemic is utilised (Pala, Güneralp & Mergen
However there is a discrepancy between the "implied" and the "actual" public awareness. The authors believe that of this discrepancy is due to the following reasons. First of all, people do not directly perceive the state of the epidemic. The situation is realised by various organizations, first. Then actions to be taken are determined and information is conveyed to the public via various educational and communication means. Thus, there is a certain time delay before the public gets to know the real situation, before people process what they learn and start taking some precautions. The clinical evolution of the disease includes certain distinct stages: HIV-negative, HIV-positive seronegative, HIV-positive seropositive unidentified, HIV-positive seropositive identified asymptomatic, Symptomatic - ie. developed AIDS, Dead. These stages are shown for each group defined above (Figure 1). HIV-negative state accumulates people that have the risk of becoming infected, namely the susceptible in the classical epidemic terminology. The newly infected people pass onto the next stage that represents the HIV-positive, seronegative individuals. The duration of this stage is three months, during which time the antigens reach a threshold level that permits the identification of the virus in the blood. This leads to the group of people who are HIV-positive seropositive, thus can be identified, but not tested yet. A third order material delay of 7.5 years, which represents the mean incubation period of the disease, is used. This is the time period between the infection and the appearance of the symptoms. Individuals that become identified at each stage of the incubation period are represented as HIV-positive seropositive identified asymptomatic. The reason for the distinction of the unidentified and identified infected people is that a percentage of the HIV-positive tested individuals no longer take part in sexual activities. Thus, they are "isolated" in the sense that they do not have effect in the spread of the virus. People that become clinically symptomatic, or in other words that develop AIDS are accumulated in the next stage. The mean life expectancy from the time of the development of AIDS is taken to be 2 years. This evolution ultimately leads to death (Pala, Güneralp & Mergen 1997).
A total of 35 runs were performed. It can be concluded from the
results of these runs that there is no threat of extinction for
the human beings even without exerting significant control on
their sexual practices. However, it should be noted that the significant
difference in the number of infected people within the next 20
years makes it worthwhile to try to increase the public awareness.
One of the major consequences of the epidemic is the reduction
in the useful lifetime of the individuals. From an economical
perspective, HIV affects people during their most productive stages
of life. This definitely can become a dominant factor that adversely
affects the GNP and life expectancy of the nation, both of which
are critical factors of economic development for the developing
The results obtained from the sensitivity analysis suggest that,
the spread of the AIDS epidemic in South-East Asia is highly sensitive
to the identification of the infected CSW and the prevention of
their sexual practices. Identification and removal of the infected
CSW greatly reduces the infection rate resulting from commercial
sex, a widespread practice in the region of study. In turn, the
reduced infection rate of heterosexual, and bisexual men results
in lower HIV prevalence among these groups. The lower HIV- positive
ratio among these groups serves as a means of survival for the
heterosexual women who engage heavily in sexual practice with
individuals from these two groups. For example, an increase in
the testing of CSW from 30% to 90%, decreases the number of total
infections by 35%. Thus, as a result of this study, we recommend
that the policy makers concentrate on the CSW; their identification
and the isolation of the identified CSW from further practice
of commercial sex.
The runs also revealed the importance of condom usage in terms
of reducing virus transmission. Condom usage should be promoted
in the society in all sexual practices, but especially in commercial
Finally, we would like to conclude with stating that the availability
of reliable and usable data constitutes the major constraint in
building and validating such models. The main line of future research
on this model should be data collection and extensive validity
Ahlgren J. David, Stein c. Alex, "Dynamic Models of the AIDS Epidemic". Simulation January 1990: 7-19.
Coaster J. Thomas, Aggleton Peter, "HIV Prevention in Developed Countries". The Lancet 1996; 348: 1143-48.
Cruz-Grote Doris, "Prevention of HIV Infection in Developing Countries". The Lancet 1996; 348: 1071-74.
Dangerfield Brian, "Modelling the Epidemiological Consequences of HIV Infection and AIDS: A Contribution from Operational Research". J. Opl. Res. Soc. 1990; Vol. 41, No. 4: 273-289
Mergen Onur, Pala Özge, "Modeling the Dynamics of the AIDS Epidemic". Bogazici University IE 550 Term Project Fall 1996.
Nelson E. Kenrad and et.al. "Changes in Sexual Behavior and a Decline in HIV Infection Among Young Men in Thailand". The New England Journal of Medicine 1996: Vol. 335, No. 5: 297-302.
Pala Özge, Güneralp Burak, Mergen Onur, "Modelling the Dynamics of the AIDS Epidemic in South East Asia", 1997.
Quinn C. Thomas, "Global Burden of the HIV Pandemic". The Lancet 1996; 348, 99-106.
Virkkunen Juhani, Hamalainen P. Raimo, "Simulation Analysis of Some Basic Hypotheses about the Spread of AIDS". Simulation January 1990: 21-28.
WHO: World Health Organization's HIV/AIDS Surveillance Reports
World Bank Annual Reports
Proceedings and Results of The
XIth International AIDS Conference
Vancouver, B.C., July 7-11, 1996
Brandeau L. Margaret, Lee L. Hau et.al. "Modeling
Human Immunodeficiency Virus Screening and Intervention: Analysis
of Screening Policies to Reduce HIV Transmission to Newborns".
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