It’s what keeps microbiologists awake at night: when the next deadly disease breaks out, modern air travel means it will be halfway round the world before we even notice.
Or does it? Mass air travel might instead mean some bad outbreaks are less likely to happen, according to an analysis that turns accepted thinking about pandemics on its head.
The idea that the world is overdue for an outbreak of a fatal infectious disease – aka “the Big One” – is so widely accepted it has become a sci-fi plot staple, and the target of intense preparedness efforts by governments. The deadliest epidemic in history, the 1918 Spanish flu, killed over 50 million people in a couple of years.
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Natural vaccination
But new diseases don’t spring up from nowhere – they evolve from related strains of viruses or bacteria, point out Robin Thompson of the University of Oxford and colleagues. The new microbe may differ from the old by only a few genetic mutations. That means people previously exposed to the first strain – thanks to air travel – may have some degree of immune resistance to the new deadly strain. So they’d be less likely to catch it, or if they do, to die from it. “It’s like a natural vaccination,” says Thompson
In other words, the continual spreading of germs around the world makes it all the harder for a microbe to evolve in isolation long enough that when it finally breaks out, it wreaks destruction. “We may have been thinking about air travel all wrong,” says Thompson.
The team carried out mathematical modelling of 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 governing the number of cases is the degree of cross-immunity between the pandemic strain and its closest relative.
Pre-existing immunity
While the work is theoretical, it could explain why we haven’t yet had that devastating flu pandemic we keep being warned about. For instance the 2009 swine flu outbreak, which was classed as a pandemic by the World Health Organization, turned out to be a lot milder than predicted. Animal research suggests this could be because lots of people had pre-existing immunity.
And some of the most devastating outbreaks in history happened to communities that were isolated for a long time before being exposed to a new pathogen by invaders, such as with the impact of smallpox on Native Americans and dysentery on Pacific Islanders.
Sadly, we can’t relax yet though. The new view doesn’t mean there will never be another deadly pandemic, says Thompson. Completely new diseases could spring up that have not been preceded by a related strain.
For instance, a microbe that currently affects animals could mutate to infect people. This is what happened when HIV spread from chimps to humans in Africa in the early 20th Century. “Evolution can throw up twists and turns that are hard to predict,” says Stephen Parnell of the University of Salford in the UK, who was not involved in the latest work.
And even though swine flu didn’t turn out to be the Big One, it still killed 200,000 people worldwide and was worrying because it mainly caused deaths in younger people, while normal winter flu generally kills older people.
BioRxiv
