Model of disease spread proves common assumption wrong.
The current Ebola outbreak shows how quickly diseases can spread with global jet travel.
Yet knowing how to predict the spread of these epidemics is still uncertain, because the complicated models used are not fully understood, says a UC Berkeley biophysicist.
Using a very simple model of disease spread, Oskar Hallatschek, assistant professor of physics, proved that one common assumption is actually wrong. Most models have taken for granted that if disease vectors, such as humans, have any chance of “jumping” outside the initial outbreak area – by plane or train, for example – the outbreak quickly metastasizes into an epidemic.
Hallatschek and co-author Daniel Fisher of Stanford University found instead that if the chance of long-distance dispersal is low enough, the disease spreads quite slowly, like a wave rippling out from the initial outbreak. This type of spread was common centuries ago when humans rarely traveled. The Black Death spread through 14th-century Europe as a wave, for example.
But if the chance of jumping is above a threshold level – which is often the situation today with frequent air travel – the diseases can generate enough satellite outbreaks to spread like wildfire. And the greater the chance that people can hop around the globe, the faster the spread.
“With our simple model, we clearly show that one of the key factors that controls the spread of infection is how common long-range jumps are in the dispersal of a disease,” said Hallatschek, who is the William H. McAdams Chair in physics and a member of the UC Berkeley arm of the California Institute for Quantitative Biosciences (QB3). “And what matters most are the rare cases of extremely long jumps, the individuals who take plane trips to distant places and potentially spread the disease.”
This new understanding of a simple computer model of disease spread will help epidemiologists understand the more complex models now used to predict the spread of epidemics, he said, but also help scientists understand the spread of cancer metastases, genetic mutations in animal or human populations, invasive species, wildfires and even rumors.
The paper appears in this week’s online early edition of the journal Proceedings of the National Academy of Sciences.