Ebola — first signs of a new stage?

In previous posts I had noted that the growth of the current outbreak of Ebola in West Africa had entered an exponential growth phase. Since then there were no new Ebola Disease Outbreak News for West Africa from WHO, but Caitlin Rivers and colleagues kindly provide a rather comprehensive data collection.

Figure 1: Ebola cases over time during current outbreak in several West African countries. Note that the vertical axis is scaled logarithmically.

The bad news is that the outbreak is still growing, and for Guinea and Sierra Leone we still have exponential growth. But there is also some potentially good news: First, two of the affected countries, Nigeria and Senegal, have reported no new cases for some time now and thus may have managed to contain the outbreak in their areas (see e.g. lower right of Figure 1). Second, in Liberia, the worst affected country, it looks as if the growth rate is slowing down, i.e. we have no longer an exponential growth there. In the logarithmic plot (lower left of Figure 1), this shows up as a development from a straight line in August to a slight curvature in September. Third, in Guinea and Sierra Leone, the gap between infections and deaths increases. This could be due to more effective medical help or to the virus becoming less pathogenic.

Big caveat: It is unclear how reliable the data is. The countries from which the data originate are in a very difficult situation and may not be able to register all Ebola cases and deaths. Thus, the development in Liberia (lower left of Figure 1) may reflect not the course of the epidemic but the collapse of the medical infrastructure.

In an earlier post I have referred to “extreme poverty” as one of the roots of Ebola. In fact, it is possible to find evidence for this, or at least a correlation between poverty and Ebola. Figure 2 shows a map of the world with countries coloured according to Gross Domestic Product (GDP) per capita. In West Africa, there is a cluster of five low-GDP countries (yellow, nominal GDP per capita between 400 US$ and 800 US$). These are also those countries where Ebola could not be contained. The only exception is the small Guinea Bissau, a country with a low GDP where so far no cases have been reported. Senegal and Nigeria, the two countries that have contained the outbreak, have higher GDPs (Senegal 1072 US$, Nigeria 1692 US$).

Figure 2: Nominal Gross Domestic Product (nominal GDP) per capita in the countries of the world – the brighter the colour, the lower the GDP. The red circle marks the region of the West African Ebola outbreak. The countries worst affected have a particularly low nominal GDP (yellow). Map from Wikipedia.

A related quantitative question is whether the size of the exponential growth rate depends on the degree of poverty. The bottom panel in Figure 3 shows that there is something like a correlation of nominal GDP per capita (based on IMF data taken from Wikipedia) and the growth rate during the exponential phase of the outbreak in each country. Liberia, the country with the highest Ebola growth rate, is also the poorest country, if we accept the nominal GDP per capita as indicator of poverty. GDPs per capita of Sierra Leone and Guinea are higher than those of Liberia, and the growth rates lower than that in Liberia. Senegal and Nigeria are better off with respect to both GDP and Ebola.

Other quantities such as the population density of the Human Development Index (HDI) do not show no clear correlation (top and centre of Figure 3).

Of course, we have only five points here, and therefore these (non-)correlations have to be taken with a huge grain of salt.

Figure 3: Dependency of Ebola growth rate (vertical axes) on various indexes (horizontal axes). The Ebola growth rate is the growth rate during the exponential expansion phase of the epidemic in the respective country. Top panel: Dependency of growth rate on inequality-adjusted Human Development Index (HDI), 2013, as taken from Wikipedia. Centre panel: Dependency of growth rate on population density in 1/km^2  (taken from Wikipedia page of the respective country). Bottom panel: Dependency of growth rate on nominal GDP per capita.