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To track disease, follow the money

Using paper currency to track infections sounds like a clever idea. But we think the smart money is on your phone.
The free movement of people and the ease of modern travel means that infections can race halfway around the world before vaccine manufacturers have finished tying their shoelaces. The deadly H2N3 strain of "Aussie flu" took only 4 months to spread from the initial outbreak in New South Wales to the UK, where it is now putting a major strain on the NHS.

And once the virus took hold in the UK, the density and mobility of the population accelerated the speed of transmission.

Contrast this with the slow march of infectious diseases before the era of mass movement and jet travel. The Black Death took around 8 years to roll across Europe, though it did manage to wipe out at least a quarter of the population in the process.

The way that influenza spreads can be tracked by tools such as Flusurvey,(1) which produces rapidly-updated heatmaps of illness across the country. Public Health England (and its equivalents in Scotland, Wales and Northern Ireland) also carries out surveillance of transmissable disease and patterns of microbial resistance. Working with European and global networks such as EARS-Net and PorMED-mail, they provide an early warning of new threats like SARS and Zika.

Disease mapping software is now accessible to the public too. A new app and website called Doctors Report(2) allows anyone in the USA to enter their zipcode and see which diseases are trending in their area. The tracker gives a daily snapshot of 15 diseases including cold, flu, Lyme disease and glandular fever, and the data can be further broken down by age group and severity.

The problem is, these surveys are retrospective - they rely on reports by doctors and labs, so they are always one step behind the infection. Predicting and anticipating the spread of disease requires mathematical models that take account of patterns of mobility and human interaction. And disease transmission models are becoming more sophisticated.

A study published in Science in 2013(3) showed that the course of global pandemics can best be predicted not by geographical distances by 'effective distance', which depends on factors like flight patterns and aviation hubs. Proof of the model came from analysing the 2003 SARS outbreak and the 2009 H1N1 pandemic. When the epidemics' arrival times in different cities were plotted against their effective distances, there was a strong positive correlation. The model doesn't require any knowledge about the disease or the infectious agent, and it can also be applied to social contagions such as fads and fake news. This explains why some ideas 'go viral', while others just go fungal.

This week, the same group announced an innovative way to relate people's movements to the global spread of disease.(4) They have developed a model based on tracking the movement of US banknotes, which are carried and passed on by people in the same way as disease. The project can be followed online at a website called (5) (George Washington appears on every $1 bill). Visitors can enter the serial number of the banknotes in their wallet and see where else the notes have been. As of today, the website has tracked nearly 300 million movements across the US, covering more than $1 billion dollars.

Using filthy lucre to model the spread of disease seems strangely appropriate. Banknotes have long been known to harbour nasties; a Scientific American article last year showed that US currency was a breeding ground for microbes, including faecal bacteria, moulds and yeast.(6) Lower-denomination banknotes are more likely to be contaminated (perhaps because they are more widely handled).(7) A US government survey found that that 94% of $1 bills were carrying bacteria, including pathogens responsible for pneumonia, blood infections, diarrhoea, and urinary tract and respiratory system infections. And international research indicates that the poorer the nation, the dirtier the currency.

Analysing banknotes also reveals traces of cocaine, as rolled-up banknotes are often used to snort the drug. One bright spark in the UK suggested that contaminated cash could be used to monitor regional patterns of illicit drug use, until it was found that cocaine residue was present on virtually every note in circulation.(8) Police have stopped testing notes for traces of the drug in criminal investigations as the contamination is so widespread.

The flu virus has been shown to survive for up to 17 days on banknotes, especially when accompanied by mucus, which may make you think twice before sticking them up your nose. The polymer banknotes recently introduced in the UK are claimed to carry only one-third of the bacteria of paper currency,(9) but we can't comment on their effectiveness for intranasal drug administration.

The belief that banknotes are a vector for disease has led to a public health argument for a cashless society. But banknotes do not spread or reproduce like infectious agents, more's the pity, nor does the movement of money accurately reproduce the movement of people.

Which prompts the question, how sustainable is the idea of tracking disease through banknotes? Paper currency is dying out. People are increasingly paying by card, or digitally using contactless or cashless systems like Apple Pay.

So here's a thought. There is another item that people today carry everywhere - in fact it clings to most of them like chlamydia - and it continually logs their mobility. Their mobile phone. Why not use the GPS function on smartphones to tag people's movements and patterns of population density, and so model the spread of disease?

Unlike hard cash, mobile phones are carried by almost everyone (more people around the world have access to a mobile phone than a toilet). And tracking mobile phones doesn't depend on self-reporting. The police and intelligence organisations routinely trace the movement of individuals through their mobiles.

Wearable devices like fitness trackers can also be used to monitor people's movements. An Australian student recently stumbled across a Global Heat Map published by the GPS tracking company Strava, which records the locations and movements of 27 million people who use fitness trackers like Fitbit and Jawbone.(10) The student was able to track the jogging routes of US soldiers based in Afghanistan and Syria, which prompted a panicky directive from the Pentagon for military personnel to upgrade their privacy settings.

A map of people's movements tracked by mobile phone might look like Brownian motion - chaotic and apparently random - but if you feed in a start point and apply the right algorithms, you could quickly gain a picture of how widely and rapidly infection can spread.

This seems such an obvious idea, we would be surprised if tech companies like Google, who are aggressively colonising the health sector, are not already looking into it. If they are not, we will share the concept with them for a split of the profits. Our motivation is to contribute to progress in epidemiology, so we're not greedy; 5% would be acceptable. And cash will be just fine.

© Life Healthcare Communications 2018

3. The Hidden Geometry of Complex, Network-Driven Contagion Phenomena. Brockmann D, Helbing D. Science 2013;342:1337-42.
6. Dirty Money. Maron DF. Scientific American, January 3, 2017.
7. Paper money and coins as potential vectors of transmissable disease. Angelakis E et al. Future Microbiol. 2014;9(2):249-261.

9th February 2018



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