CRF Blog

The Ebola crisis

by David De La Torre

In Much worse to come, The Economist reports that the Ebola epidemic could get worse in Africa.

WHO fears it could see between 5,000 and 10,000 new cases reported a week by the beginning of December; that is, as many cases each week as have been seen in the entire outbreak up to this point. This is the terrifying thing about exponential growth as applied to disease: what is happening now, and what happens next, is always as bad as the sum of everything that has happened to date.

Exponential growth cannot continue indefinitely; there are always barriers. In the previous 20 major outbreaks of the disease since its discovery in 1976, all of which took place in and around the Democratic Republic of the Congo, the initial rapid spread quickly subsided. In the current outbreak, though, the limits have been pushed much further back; it has already claimed more victims than all the previous outbreaks put together.

There are two reasons for this. Those earlier outbreaks were often in isolated places where there are few opportunities for transmission far afield — the transfer of the virus between a wild animal and a human that sets off all such outbreaks is more likely off the beaten track. And they were mostly recognised quickly, with cases isolated and contacts traced from very early on; one was stopped this way in Congo in the past few months. The west African outbreak has broken through the barriers of isolation and into the general population, both in the countryside and the cities, and it was up and running before public-health personnel cottoned on. There is no reason to expect it to subside of its own accord, nor to expect it to come under control in the absence of a far larger effort to stop it.

Trying to be precise about how bad things could get, absent that effort, is not possible. This is not just because the actual number of cases is not well known. The rate at which cases give rise to subsequent cases, which epidemiologists call R?, is the key variable. For easily transmitted diseases R? can be high; for measles it is 18. For a disease like Ebola, much harder to catch, it is lower: estimates of R? in different parts of the outbreak range from 1.5 to 2.2. Any R? above 1 is bad news, though, and seemingly small differences in R? can matter a lot. An R? of 2.2 may sound not much bigger than an R? of 1.5, but it means numbers will double twice as fast. [more]