Lately there is a number of questions about the efforts to screen passengers and ban air travel. Will these efforts be sufficient?
Passengers are getting screened who are coming from countries with Ebola (Guinea, Liberia, and Sierra Leone). This is being done both via laser thermometers (a good ideas since this doesn’t require touching anyone) as well as through questionnaires. There are a couple of gaps though with this approach.
First, one must remember that the incubation period for Ebola can run up to 21 days. This is the period of time which a person can be harboring a virus but not show any signs and/or symptoms. A fever is a telltale sign, but unless an individual is three weeks past an exposure to Ebola, this may not be effective in identifying cases. Using a fever alone as a screening tool would not have caught the first case that was diagnosed in the United States. He arrived here on 9/19 but didn’t have symptoms until 9/24.
The questionnaire is meant to identify other risks that may indicate that someone has had Ebola exposure. Like all screening measures, this has some holes as well.
When conducting surveys, there is a common problem with something called recall bias. This is simply because human memory fades both with physiological age but also as events become more distant in time. For example, could you remember what you had for lunch and portion sizes five days ago? What about 21 days ago? These are simply errors that are introduced through poor recollection.
Another problem with screening questionnaires though which is even a bigger issue for this problem is response bias. This is the instance where people will lie in order to achieve their desired outcome, in this case passage on an airline or into a country. This could simply be motivated by the desire to see friends or family or for wanting to conduct business. However, it is not outside of the realm of possibility that someone might knowingly want to bring a disease that they are harboring into another country as sort of a crude human biological weapon.
These screening measures are good, but there are clear gaps. They will be implemented at the five airports (New York JFK, Washington Dulles, Newark, Chicago O’Hare, and Atlanta) with the majority of travelers coming from affected countries. Roughly 150 people enter the US each day from the affected countries and ALMOST all of them enter through these airports.
What about those who come in through other airports? That is a distinct gap. Some people have suggested that we should ban all flights from affected countries. However, that approach has problems as well.
First, airlines are not the only means by which to exit a country. There are land and sea routes as well. Someone who was set on getting to a destination would simply find the path of least resistance into a bordering country and then begin air travel from that location. We’ve already seen something similar happening when some Liberian men were trying to illegally enter the US through Costa Rica. General John Kelly, the commander for South American operations for the Pentagon, raises a very big concern about the problems that will arise if Ebola gets established in South America. People will begin fleeing north to seek medical care in the US. That creates a massive illegal border crossing situation along the Mexican border.
There are even bigger problems with banning air travel. How do aid workers get in and out of a country to help stop the problem at its source? This is one of the big questions I’m asking as I make my decision about whether to go help. I want to know with very little uncertainty that I will be able to leave once my time in field is over. I do not want to get trapped like I’ve entered some massive diseases black hole.
Worse, banning air travel could destabilize already shaky governments in those regions. If that happens, Ebola could spread much more quickly in those areas and be even more difficult to contain, leading to a possible pandemic. That is something we DEFINITELY do not want to see.
There is also a bigger problem emerging now that Ebola has shown up in Lagos, Nigeria. It is the biggest city in Africa with an estimated population of 21 million. It is also an obvious air travel hub due to it’s size. If Ebola gains a foothold in Lagos, we might be too late because of the volume of air traffic through that city.
Air travel from areas hardest hit by Ebola from Gomes MFC, Pastore y Piontti A, Rossi L, Chao D, Longini I, Halloran ME, Vespignani A. Assessing the International Spreading Risk Associated with the 2014 West African Ebola Outbreak. PLOS Currents Outbreaks. 2014 Sep 2. Edition 1. doi: 10.1371/currents.outbreaks.cd818f63d40e24aef769dda7df9e0da5.
Clearly, we have much more to do.
10/15/2014 Addendum:
Back in August, I had started reading Linked: How Everything Is Connected to Everything Else and What It Means for Business, Science, and Everyday Life by Albert-Laszlo Barabasi on network science and theory because I thought it would give me some insights into how outbreaks behave. Little did I know that this would be useful so soon.
Without going into the technical details, airports are scale-free networks. Essentially, in the real world this looks like larger airports with major airline hubs, significantly more medium size airports with much less commercial air traffic, and finally many small municipal airports with very little major air carrier activity at all. It’s a classic example of this type of network.
I’m going to quote from the book because what he says is important.
“The accidental removal of a single hub will not be fatal either, since the continuous hierarchy of several large hubs will maintain the network’s integrity…Amazingly, most networks of interest, ranging from the Internet to the cell, are scale-free and have a degree exponent smaller than three. Therefore, these networks break apart only after all nodes have been removed–or, for all practical purposes, never.”
This is why closing air travel to a particular country having an outbreak would not likely make much of a difference. Our travel networks are robust. Between land, sea, and air, there are many alternate routes for people to travel with whatever microorganisms they may harbor. I think the way that this applies in this case is that the only way to stop spread via transportation hubs would be to stop ALL transportation hubs in a much broader scale than to that of a few countries. That is simply not economically and possibly ethically feasible.
It’s easy to want to say that we should do that in all of the affected countries. However, I think people would have a hard time making that kind of statement when it affects their own, as it has now in the US.
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