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Ebola Is Coming. A Travel Ban Won't Stop Outbreaks
Air traffic connections from West African countries to the rest of the world (Image CC BY 4.0: Alessandro Vespignani / PLOS Currents Outbreaks)
Ebola has officially gone global.
The World Health Organization recently confirmed that a Spanish nurse was the first case of transmission outside Africa. Now it seems the first patient diagnosed in the United States transmitted the disease before he died.
More outbreaks are on their way.
While nations struggle to contain the epidemic in West Africa, other countries are discussing how to protect their own citizens, with governments and health authorities repeatedly asked the same question:
Why don’t we just ban flights from Africa?
The idea seems logical. Prevent sick people entering the country, keep your loved ones safe. It’s selfish, but understandable. A survey of over 1000 people by NBC News found that the majority of Americans (58%) support a ban on flights from countries where the Ebola virus has broken out.
Dr Tom Frieden, director of the US Centers for Disease Control and Prevention, has tried to explain why he doesn’t support a travel ban:
Importantly, isolating countries won’t keep Ebola contained and away from American shores. Paradoxically, it will increase the risk that Ebola will spread in those countries and to other countries, and that we will have more patients who develop Ebola in the US. People will move between countries, even when governments restrict travel and trade. And that kind of travel becomes almost impossible to track.
Simply put: you can’t seal the country. If you blocked air travel, it would force desperate individuals to use alternative routes – over land and sea – to escape the epidemic. They’ll still end up in the US, except you won’t know where.
An attempted travel ban would be like locking yourself in a cabin on a sinking ship and praying the flood doesn’t seep through the gaps, and that the water pressure won’t be enough to burst through the door.
There are many reasons why a flight ban would be practically impossible to implement. For example, remember that Thomas Eric Duncan, the US patient who caught the Ebola virus in his native Liberia, flew to Texas via Brussels in Belgium. An effective ban would require international coordination. Would every nation agree to quarantine West Africa, to cripple their economy and choke them of humanitarian aid? Unlikely.
But for the sake of argument, what happens when you reduce air travel?
Air traffic reduction
Professor Alex Vespignani, a physicist at Northeastern University in Boston, MA, has developed a computer model that predicts how air traffic affects the spread of Ebola.
His team at the Laboratory for the Modeling of Biological and Socio-technical Systems used a high-resolution map of human populations (3300 locations in 220 countries) and added daily airline passenger traffic. This model considers connecting flights and final destinations, plus details of the disease dynamics, such as incubation time of the Ebola virus and the fact a susceptible individual can only be infected by someone who shows symptoms of illness.
“All the people who have been exposed to the disease but are not yet in the symptomatic state can in principle travel,” says Vespignani. “So since we have this model that puts people on a plane, we can assess the probability of getting an infectious individual in countries around the world.”
Air traffic connections is a key factor influencing the chances of importing a case of Ebola. Over 6000 passengers normally flow into the United Kingdom every week, while the US and Ghana each receive over 3000 travellers (see image at the top of this page). The nations affected by the epidemic have urban areas with international airports, or are connected to West Africa’s travel hub, Nigeria, which has had one outbreak of 20 cases from a single importation from Liberia.
Vespignani’s computer model simulates a virtual world in which billions of individuals move around, come into contact with one another, and potentially spread disease. The aim is to predict cases like that of Thomas Eric Duncan.
Countries ranked by risk (relative probability) of importing a case of Ebola by 31 October. Red bars are nations that have already experienced case importation. LEFT: No air traffic reduction (ATR) reflects travel before the 2014 epidemic in West Africa. RIGHT: 80% ATR approximates the current reduction in air traffic to and from countries with Ebola. (Image: Alessandro Vespignani / www.mobs-lab.org)
The model calculates the risk of importing at least one Ebola case after running millions of simulations. They’re run under two scenarios: no air traffic reduction (ATR) to mirror travel before the epidemic, and reducing air travel by 80% to reflect airlines suspending flights and passengers avoiding travel.
The number of simulations in which a virtual country ends up with an outbreak gives a statistic for the risk of importing an Ebola case in the real world. So if a country gets the disease in half of them, the probability of case importation is 50%. That’s the prediction in October for Ghana, which lies between the affected nations – Guinea, Liberia and Sierra Leone – and Nigeria.
Big risks
For most countries, the results indicate that an 80% air traffic reduction more than halves the probability of importing a case of Ebola. For the US, the risk is reduced from around 75% to 25%.
But those risks don’t stay static.
An 80% reduction in air traffic only postpones the inevitable. “This is just delaying by four weeks what would have happened without those travel restrictions,” Vespignani explains. What about a 90% reduction? It would only buy you another month or two.
Like weather forecasts, Vespignani’s virtual model is calibrated using real-world data. As conditions change, the model is revised and simulations are re-run. To make accurate predictions, it needs to be regularly updated with the number of cases and deaths at each geographical area. Like weather, there’s higher confidence in forecasts for next month than further into the future.
The predictions above are for October, calibrated from recent data. In the original study, the model was calibrated with data from 6 July to 9 August to predict how an 80% air traffic reduction affects risks for September. The results showed that outside Africa, the risk was tiny – under 5% probability for every country except the UK, which has the most connections. (England’s chief medical officer says the UK should expect a handful of cases.) A dozen countries have since joined the UK with a risk over 5%.
As the number of Ebola cases continues to rise in West Africa, so too will the risk of case importation. “We’re a little safer for a finite amount of time, but then you are not really solving the problem,” says Vespignani.
Small outbreaks
The forecasts aren’t all doom and gloom though.
As well as modelling the global spread of Ebola, Vespignani’s simulations also predict local transmissions within a community, in hospitals and at funerals. And the numbers for secondary infections from imported cases are reassuring.
“These outbreaks should be very, very small – 2 or 3 cases,” he says. “I won’t panic if tomorrow we hear that in Texas there is another case. This is totally normal.”
[Note: A prophetic quote, given that I interviewed Vespignani before it was revealed Thomas Eric Duncan had transmitted the virus.]
Projections for outbreak size (number of cases) after a country imports Ebola. LEFT: 1 September. RIGHT: 22 September. (Image CC BY 4.0: Alessandro Vespignani / PLOS Currents Outbreaks)
One thing that computer simulations can’t predict is human error. In the two cases of person-to-person transmission outside Africa (the Spanish nurse and second US patient), there might have been a breach in proper safety protocols.
“But these mistakes are very rare, and again this is not going to give rise to large outbreaks,” says Vespignani. “Obviously what is happening in Liberia, Sierra Leone and Guinea is something that is of a totally different scale, with a healthcare system that we cannot even think of in our countries.”
Vespignani is confident that the healthcare systems in Europe and North America are strong enough to stop outbreaks from ever reaching epidemic proportions, but says Asia is another matter. “If you ask me about India, China, other countries, then there are a lot of question marks.”
Worse for the world
An Ebola epidemic in two countries with a combined population of 2.6 billion is not only terrifying, it further highlights the futility of attempting a travel ban. Could the US ban all flights from Asia and Africa? Where would it end, isolating the North American subcontinent from the rest of the world?
A travel ban is short-sighted, and would be ineffective in the long run. It’s the epidemiological equivalent of an ostrich sticking its head in the sand: ignore the problem and hope it goes away.
And the Ebola epidemic isn’t going anywhere. It’s actually getting worse: the number of cases in West Africa continues to increase at an exponential rate.
Projections based on current trends using a dozen different models give future figures in the same ballpark: WHO predicts the total number will exceed 20,000 by 2 November, for example, while Vespignani’s simulations say 18,100 to 55,400 cases by the end of October.
Projection for the total number of Ebola cases in West Africa by 31 October. Red circles are reported cases. Gray area is the range of projected cases, based on a worst-case scenario where the epidemic continues to grow exponentially. (Image: Alessandro Vespignani / www.mobs-lab.org)
According to a projection by the CDC, by late January 2015 there could be up to 1.4 million cases in West Africa alone.
War on Ebola
As Vespignani’s computer simulations show, Ebola can easily spread across the globe. “This epidemic has pandemic potential,” he warns. “What happens next year depends on what we are able to do in Africa. If we win this battle, it’s okay. If we lose the battle there, then this thing is serious.”
The only way to stop Ebola going truly global is to beat the epidemic in West Africa. Governments get this: the US is deploying 4000 troops to Liberia and the UK is sending 750 soldiers to Sierra Leone. Nonetheless, according to the NBC survey, over half (51%) of Americans disapprove of sending US troops to fight the spread of Ebola.
The survey also revealed that most Americans (72%) understand that the Ebola virus is transmitted via contact with bodily fluids, which suggests that health authorities like the CDC and WHO have successfully educated the public on how the disease spreads from person to person.
Calls for a travel ban illustrate that there’s yet another battle to be won over Ebola: explaining how the disease spreads between populations.
JV Chamary is a biologist and writer – read more of his stories on Forbes and follow him on Google+ and Twitter
