A friend recently asked us whether the massive Ebola outbreak in West Africa could be regarded as a “black swan” in the sense of Nassim Taleb’s definition of the term. After thinking it over, we concluded that yes, it can definitely be characterized as one. Evidently, something is very different about this year’s outbreak compared to previous ones, and a number of unexpected developments have occurred. Chief among them is that a hitherto firmly held belief had to be abandoned. It was thought that the very thing that that makes the illness rather terrifying, namely its high mortality rate, helped in containing outbreaks.

We can definitely state that the current outbreak is anything but “well contained”. Below is a statistical table that shows all Ebola outbreaks since the discovery of the disease in 1976. Note that this graphic is already dated by now – the 2014 event has literally “gone off the chart” in the meantime. Even so, this graphic gives a good impression of how small the previous incidences of Ebola outbreaks were by comparison.

From a statistical viewpoint, the 2014 outbreak definitely must be regarded as a “black swan” – it was hitherto held to be impossible for the illness to propagate in such fashion (source: news.au.com)

Another way of looking at the “black swan” quality of the current outbreak is its geographical spread. All previous Ebola outbreaks were confined to a few isolated locations at most, mainly because they occurred in remote villages in the bush. As a result sick (and therefore infectious) people simply didn’t manage to reach any other villages to spread the virus further. Moreover, since also many of those who catch the illness quickly die, the virus was thought not to propagate very easily. Death is obviously the ultimate impediment to mobility (the dead do however remain infectious for quite some time).

Geographical distribution of Ebola in West Africa as of October 7 to 8, according to a recent WHO report – click to enlarge.

The fact that the outbreak already has “black swan” qualities makes it more likely that a few other strongly held beliefs could also turn out to be wrong. There is already an intense debate over how the virus actually moves from person to person. Given that it is present in sputum, a number of virologists have stated that if one were for instance bathed in a gentle spray of saliva emitted by a coughing and sneezing person that has been infected, one will probably catch it. In fact, a recent warning issued by the Center for Infectious Disease Research and Policy is mainly noteworthy for its admission regarding the uncertainties about possible transmission vectors. It recommends that health care workers be fitted out with proper respirators to ward off infection via aerosol particles.

Before hearing about this, we remarked as follows in a recent email conversation: Even considering the low standards of hygiene and certain cultural idosyncrasies that make it more likely for the disease to spread in African countries, it seems not as difficult to get infected as was generally held. One must also keep in mind that the official numbers almost certainly understate the number of infected people by a fairly big margin – many people reportedly get infected and simply die without ever making it into the statistics.

The progression of the outbreak shows that many hitherto widely accepted nostrums about Ebola and the likelihood of it spreading beyond a fairly small group of people have proved wrong. There is therefore possibly one more article of faith that may prove wrong as well, namely that there is no reason to worry that it could spread in developed countries.

What if it did? A graphic in the Washington Post seems to suggest that even in the developed world medical resources would be quickly overwhelmed. Obviously in West Africa, the situation is well beyond the “worrisome” stage by now.

Ebola containment – definitely not an easy task – click to enlarge.

One thing that is favorable from the point of view of citizens in developed countries is that the authorities are by now definitely on red alert. However, all it will take is one small slip-up and the genie will be out of the bottle.

Meanwhile, the projections for the expected progression of the outbreak in West Africa are positively horrifying. Below we show a number of recent statistics.

Ebola Outbreak, Statistics as of Early October 2014

The number of known cases by now exceeds 8,300 – click to enlarge.

Total cases versus mortality – the fatality rate is at approx. 48% so far. In this respect, some of the previous smaller outbreaks have been worse, but it is probably a good bet that if they had become similarly large, the fatality rate would have been similar as well – click to enlarge.

This projection shows – in light blue – what is going to happen over the next four weeks if no change in control efforts occurs. This projection was worked out with a healthmap model – you can look at the projections with improved control efforts as well as those with a deterioration in control efforts here (under the deterioration assumption, about 3,000 more cases would be recorded over the next four weeks than shown on the graph). As you can see, the disease seems likely to continue to spread in exponential fashion. However, the next chart suggests things may actually be slightly improving now – click to enlarge.

A small light at the end of the tunnel seems to be suggested by this chart, which shows that the number of cases reported weekly in West Africa is declining somewhat recently. While this is hopefully the result of mitigation efforts, it could also be due to the local authorities simply getting overwhelmed. One big problem is also that the disease has by now spread into the DCR and Nigeria. Although the number of cases there remains small so far, this is very worrisome given the population density in big cities like Lagos – click to enlarge.

This statistic shows that in terms of deaths per day, Ebola continues to compare quite favorably to a whole host of other diseases that receive a lot less attention – click to enlarge.

Considering the last chart above, one may well wonder why there is such a lot of attention paid to Ebola given that many other diseases are exacting a far greater death toll. The explanation is of course simply this: No other major disease has a comparable fatality rate that kills so many of the people it infects so quickly and in such a horrible manner (the symptoms of the disease are such that people suffer a great deal before they either recover or die). Moreover, Ebola is the only disease that is so far considered entirely untreatable (although promising experimental stage vaccines luckily exist already). Note here that only a close relative of the Ebola virus, the Marburg virus, has an even worse fatality rate.

Furthermore, since the virus has now infected a great many human hosts, it may be mutating into something even more dangerous (there are a number of ways in which it could become more lethal, although as far as we are aware, major changes in how people can get infected are still deemed unlikely by experts).

Conclusion:

Hopefully the outbreak in Africa can be brought under control soon and the negative surprises characterizing the epidemic are not going to be added to. In short, the black swan better not get any blacker.

A picture of the Ebola virion. The virus is unusually large and has a very typical shape.

Health workers carrying a dead Ebola victim from a house in Monrovia

(Photo © dpa)