Aircraft travel is a wonderful thing for an epidemic looking to find new scenery and new susceptible populations. That’s reasonably self-evident and a rich source of fuel for a paranoid imagination. It’s why we have mostly ineffective checkpoints at airports for travelers to have their temperatures taken and why we have dudes in biohazard gear picketing in front of the White House demanding STOP THE FLIGHTS.
The mechanics of long-range jumping are less understood, but a team of physicists from UC Berkeley just published a paper describing how the precise frequency of such jumps can turn a localized outbreak into a full-on global epidemic. It doesn’t take much.
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The old way for an epidemic to spread, according to the paper, is pancake-like or wave-like, a steady outward march that’s geographically roughly circular in shape. This is how the Black Death spread, expanding outward at a steady clip of 300 to 600 kilometers per year. This sort of spread is considered continuous (like a wave or curve), yet an epidemic in the age of jet travel adds in discontinuous or discrete jumps.
The pre-airliner version of an outbreak would be like getting a weekly epidemic paycheck, always the same and always on-time, while long-distance travel delivers disease as huge, random lottery winnings, only instead of millions of dollars, its hemorrhagic fever.
So, while the outward, continuous spread might still occur locally, the larger picture is dynamic and stochastic. That is, it’s given by relatively random leaps dictated by probability rather than determination. “Potentially, even very rare jumps over exceptionally large distances could be important,” the paper notes. “If this is the case, then the stochastic nature of the jumps that drive the dynamics will be essential.”
The spread of an epidemic is often given in terms of the SIR epidemiological model, in which the rate at which an outbreak moves through a population becomes tempered by a decreasing size of the segment of the population that can be considered susceptible. This makes the outbreak’s development linear, rather than exponential.
What the current study shows is that, given long-distance travel, that dynamic is minimized, leaving growth to continue as exponential. That’s a bit disquieting, but it doesn’t lend any more credence to the STOP THE FLIGHTS crowd. The number of jumps that a local outbreak needs to become a global one are small enough to suggest that screening may ultimately be ineffective or a way to delay the inevitable.
