Simulation model to lay bare trail of avian flu, SARS and HIV/AIDS

Dresden 29 June 2006Dr. Theo Geisel and his colleagues D. Brockmann and L. Hufnagel, from the Max Planck Institute for Dynamics and Self-Organization and from the Bernstein Center for Computational Neuroscience in Göttingen, are busy with the task of globally predicting the spread of epidemics such as avian flu, SARS and HIV/AIDS. They have developed a model to forecast of the spreading of life-threatening diseases.

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Dr. Geisel first offered an overview of the diseases that are causing us to be careful and take preventive measures. The first one is H5N1, better known as avian influenza A or "bird flu". There currently is a possible emergence of a noval human influenza A virus with pandemic potential. The second one is Severe Acute Respiratory Syndrome or SARS, caused by the corona virus. It started in China in February 2004 and has had a wordwide outbreak in over 30 countries. Third, there is the combined danger of HIV/AIDS and tuberculosis with 3 million casualties per year due to HIV and 2 million deaths due to tuberculosis. Since World War One approximately 150 millions deaths have occurred which represents more total deaths than in all wars in the passed 2000 years, according to the speaker.

The travelling habits have changed and everything has been speeding up since. This has important consequences for the spreading of diseases like bird flu, SARS and HIV/AIDS. Therefore it is important that the strategies to be taken can also be simulated, stated Dr. Geisel.

The speaker also provided a brief history of pandemics. The two most important occurrences of disease spreading are the black death in the 14th century - the so-called bubonic plague occurring from 1347 to 1350 - and the spanish flu in World War One.

The bubonic plague was caused by a bacterium passed on by the oriental rat flea. It caused a case fatality from 35 percent to 100 percent. From 1347 to 1350, 20 million people died, which represents 25 percent of the entire population at that time. The time from infection to death typically was one to seven days, as Boccaccio is referring to in his Decamerone.

The spatio-temporal dynamics of epidemics are captured in the SIR model, explained Dr. Geisel. The kinetic model from being susceptible over being infected to staying infected or recover can be caught with mean field dynamics.

Random walks and diffusion are performed with a central limit theorem, as stated by the speaker who showed how diffusion works with the Galton Board 1 using balls and bins. Each ball performs N choices for deltaXn and equal probability is being achieved for the left and right way. The position is identified after N steps. The balls are collected in bins and the number of balls per bin can be defined.

Dr. Geisel expanded on the question whether diffusion constitutes an adequate model. For the epidemics in the middle ages, the answer is yes but in modern time for diseases such as SARS and HIV/AIDS we need new models because of the modern expansion of travel.

If we take the 500 largest airports with 2 million flights, there is an account for 95 percent of international civil air traffic with over 150 different aircraft types. Thanks to this data, a map of aviation movements can be created which can be used for the simulations. The simulation results differ when simulating an outbreak in New York or one in London, as Dr. Geisel showed. Different regions are being affected.

The team also uses Lévy-Flights and superdiffusion models. The question is whether we can determine human dispersal on all scales. The idea is to follow the money. The on-line bill tracking system was initiated in 1998. The money bills are marked and repeateldy reported at web sites.

The dynamics for short time trajectories have also been calculated. However, the picture is incomplete with the current predictions of the model. There has to be a comparison between theory and empirics. The results should still become more realistically, Dr. Geisel concluded.


Leslie Versweyveld

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