Travelling epidemics

Human mobility patterns and their impact on the spread of epidemics

August 31, 2011

In a globalised world, infectious diseases such as SARS, swine flu or seasonal influenza can be transmitted over the entire planet by travellers. To enable a more effective response to this threat, scientists are trying to predict the propagation pathways and speed of such pandemics. Scientists at the Max Planck Institute for Dynamics and Self-organization (MPIDS) in Göttingen and at University of Göttingen, Northwestern University and Massachusetts Institute of Technology (MIT) in the USA, have now, for the first time, managed to develop mathematical models which account for individual mobility patterns. Not only did new calculations confirm that earlier models had significantly overestimated the speed with which diseases are propagated. The previously known criteria for a global outbreak also had to be broadened. The new study was selected by the American Physical Society for publication in the first issue of its new high-profile journal, Physical Review X.

“Our calculations show that an increasing individual mobility will not – as previously assumed – result in an ever increasing propagation speed of a disease”, Geisel says. In the end, a higher number of trips will not change the fundamental character of the networks. Whether a person makes the journey to the workplace once or twice per day will hardly affect the propagation speed of the epidemic. This is why earlier models significantly overestimated the spread of epidemics.

Propagation speed of an epidemic that begins in Los Angeles, calculated with the new (top) and older (bottom) model. The density of infected individuals is shown in different colours: Red indicates a lot of infected people, blue of only a few. The new model shows a significantly lower rate of spread.

Recent findings also concerned the criteria which need to be fulfilled in order for an epidemic to hop from one population to another, that is, from one continent to the other. “Previously, it was thought that only the frequency of long trips matters”, Brockmann says. It is now clear that another factor is more significant: the duration of trips. It is not until the average duration of a trip exceeds a certain value that an epidemic may become a global pandemic. “Effectively, what matters is how long you are travelling for, not how often you travel”, the scientists say.

With this scientific article, the American Physical Society (APS) ushers in a new era of open access publishing. The new study was selected by the APS for publication in the first issue of its new open access journal, Physical Review X. In contrast to conventional scientific journals, these journals make scientific work available on line to all readers, free of charge. The work of Belik, Geisel and Brockmann, together with four other articles published in the first issue, will thus be given great exposure, as the scientists have used methods from theoretical physics to gain new fundamental insight into the propagation of diseases.

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