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How pandemics spread

Fri, 13th Dec 2013

Chris Smith

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Air passenger links between cities internationally are the most powerful determinants of disease spread, new reserach has How air travel spreads diseaseshown.

Historically, the spread of infectious diseases tend to be limited by geography, because people moved on foot or on horseback, which imposed constraints on the speed at which an illness could move through a population.

This meant mapping and mathematically modelling transmission of diseases like the Black Death, albeit based on unconfirmed data, was relatively simple.

But modern-day means of travel throw a spanner in the works because, while two cities may be on opposite sides of the world geographically, in air travel terms they can be considered closer to one another than another settlement just up the road from one othe them.

Now scientists have developed a new way to address this problem using a mathematical model that can more accurately predict the spread patterns of diseases internationally and even pinpoint the origins of disease outbreaks.

Writing in Science, Humboldt University, Berlin, scientist Dirk Brockmann and Swiss Federal Institute of Technology, Zurich, researcher Dirk Helbing have come up with what they called the "effective distance" Deff between any of the more than 4000 airports internationally.

To do this they developed a formula that takes into account both the infectivity of a given disease and how long a person remains infectious, as well as the proportion of any given population that are travelling through an airport (there are about 10 million passenger journeys per day, globally) and how "connected" that airport is to other airports based on passenger flow.

This means that a remote, but heavily served airport behaves as though it is closer to an outbreak source than a smaller, yet much more proximal airport.

This data can be represented graphically by placing any given airport at the centre of the page and plotting other airports around it in concentric rings according to the "effective" rather than geographical distance from the source airport. Plotted like this, disease outbreaks appear as concentric rings that progressively spread outwards to the more distant airports.

This approach means that Brockmann and Helbing can model where diseases, starting anywhere, are most likely to go and how quickly. To test their approach, they simulated the 2003 SARS oubreak as well as the 2009 emergence of swine 'flu using their system.

In both cases, their predictions produced a near straight-line relationship between the time when either of the two pandemics arrived at any given location and the effective distance (Deff) for that location computed by the new model.

This new work should make it easier to predict disease risks and also to test strategies designed to minimise the spread of emerging pandemics.

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Probably local public-health practices regarding quarantine, emergency department protocols etc. would have to take into account the vastly different practices of the far-off public-health regime that is closer than it appears in the mirror. grizelda, Mon, 16th Dec 2013

Not exactly new news - it's been the subject of several TV plays and documentaries.

Quarantine is a problem. Thanks to the ban on smoking, aircraft now have 3 air changes per hour instead of 5 (it saves fuel!) so respiratory infection will spread rapidly among passengers and you can reasonably assume that 10% of all passsengers will be infected at the end of a 14 hour flight with one source. Now the whole point of flying is to get there quickly, so a 3 day quarantine at each end of a 10 day holiday, or worse still a 2 day business meeting, isn't going to be popular.

Emergency department protocols can be as sharp as you like, but of the 300 people who land at Heathrow every minute, half aren't going to stay in London (where they can infect another 10,000,000 people) but will disperse to other cities and countries within a few hours. And possibly half of those who boarded the plane in Singapore  actually began their journey somewhere else.

Even if you could trace a tertiary infection in Gateshead to the primary dead chicken in Ulan Bator, it doesn't help you identify, isolate and  treat all the other cases - it's a purely academic exercise. alancalverd, Mon, 16th Dec 2013

The transmission of disease through travel is not a new thing. 

The "New World" was apparently decimated by "Old World" diseases.  And the "New World" may have donated disease having to do with fidelity to unwary travelers. 

One of the worst flu outbreaks, however, occurred at the end of World War I, and prior to the widespread use of air travel.

Travel and air travel certainly has made disease transmission more complex. 

Without travel, I wonder if the cold and flu might pop up sporadically, perhaps once every few years.  Now, it quickly spreads around the world.  The Swine Flu, for example spread around the world in a matter of months.  Could it have popped up, then locally died out a few centuries ago?

Some diseases such as Malaria don't have person to person transmission, but require a vector.  While many mosquitoes outside of the tropics are capable of transmitting Malaria, the disease dies out in the north every winter.  CliffordK, Mon, 16th Dec 2013

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