One of the fears of epidemiologists has been that cheap air travel will result in the unstoppable propagation of novel, deadly viruses, which will one day result in far too many of us dropping in our tracks. It hasn’t happened yet, and the reason may not be what you’re expecting, as NewScientist (10 November 2018, paywall) reports:
But new diseases don’t spring from nowhere – they evolve from related strains of viruses or bacteria, says Robin Thompson at the University of Oxford. The new microbe may differ from the old by only a few genetic mutations.
That often means people exposed to the first strain may have some degree of immune resistance to the new, deadlier one. This makes them less likely to catch it or, if they do, to die from it. And, thanks to air travel, that is likely to be the case around the globe. “It’s like a natural vaccination,” says Thompson.
In other words, the continual spreading of microbes around the world through air travel makes it harder for one to evolve in isolation long enough that when it finally breaks out, it kills large numbers in populations with no immunity at all. “We may have been thinking about air travel all wrong,” says Thompson.
To test the idea, he and his colleagues mathematically modelled the factors that affect the spread of a theoretical new and highly virulent microbe in a world with mega-cities and mass air travel.
They found that a crucial variable is the degree of immunity to this strain that has been gained by exposure to similar, less harmful viruses or bacteria (BioRxiv, doi.org/cwm2).
The analogy may not be strong, although recall that the early, pioneering vaccinations for smallpox (variola major and minor) were derived from the closely related, but not nearly as dangerous, disease cowpox.
But while it lowers the odds of a massively deadly outbreak, it doesn’t zero them out. The Spanish Influenza of 1918 would seem to be proof of that. All it requires is an incubation period of a few days combined with a terminal outcome in a high percentage of patients, and the influenza provides the latter.
