How Deadly Is Ebola?
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What is the main topic discussed in this episode?
Thank you for downloading More or Less from BBC Radio 4. Here's Tim Harford.
Hello and welcome to More or Less, the programme which considers itself the sequined feather boa of statistics and who are we to argue? In this week's programme, we have cyclists running over guide dogs and prisons full of sex offenders. But first, if you've been listening to the news, you could be forgiven for feeling very worried.
Doctors at the Royal Free Hospital in Northwest London have begun treating a British nurse who caught Ebola in Sierra Leone. The head of the World Health Organization has warned that the outbreak of the deadly Ebola virus in Western Africa is running out of control. Ebola is probably the most contagious and most deadly virus in the world. The strain can kill up to 90% of its victims. Most of those who get it will die through uncontrollable bleeding.
Ebola, a viral disease, has spread across West Africa, affecting Guinea, Liberia, Sierra Leone and Nigeria. Another separate outbreak has just been confirmed in Central Africa. Now it's come to the UK. William Pooley was a volunteer nurse who contracted the virus while treating patients in Sierra Leone. He was flown to London for treatment. The latest figures from the World Health Organisation showed that over 1,500 people have already died, making this the largest ever outbreak. Even worse, the WHO says that its scale has been vastly underestimated. It seems like the stuff of a Hollywood pandemic. But more or less listener Max Cousins is wondering whether Ebola is quite as terrifying as it seems.
A widely reported statistic is that Ebola kills up to 90% of those infected. I've been wondering what this statement actually means. Is it that 90% die in some areas and less than 90% in others? If so, to what extent and why do mortality rates vary? Yes, I wouldn't be totally sold by a retailer who offered up to 90% lower prices. The up to. It's a bit weaselly. Well, the stakes are a lot higher here. So what does up to 90% fatal actually mean? Ruth Alexander's here. Hello, Ruth.
Hi, Tim. What we're talking about here is the case fatality rate, which is the ratio of people diagnosed with an illness to the people who die. So let's take a fictional disease, dragonpox from Harry Potter. If 100 people were diagnosed with that and 30 of them died, then the fatality rate, the CFR, would be 30%.
We asked Maimuno Majumda, a biostatistician and epidemiologist at MIT in the United States, what the WHO means when it talks about a fatality rate of up to 90%.
That 90% figure actually comes from one outbreak of Ebola, which took place in the Congo between 2002 and 2003. So it's the highest rate we've ever seen. We've also seen CFRs much lower than that in previous outbreaks. And I think that's something important to keep in mind. a CFR of 90% could be considered the upper limit of outbreaks caused by Zarebola virus. The cumulative average, considering all Ebola cases and deaths from 1976 to present, is actually closer to about 60 to 65%. And the current outbreak thus far has a CFR of about 54%, though it's subject to change as the outbreak wanes on.
Maimuna Majumda, and since we spoke to her, the fatality rate has dropped to 52%.
And that's the average for the whole outbreak, but the fatality rate varies for each individual country. It's ranging at the moment from 42% in Sierra Leone to 66% in Guinea.
Partly that's just chance, but a lot of the variation is no accident. Here's Maimuna Majumda again.
the two biggest factors that are influencing case fatality rate are first, familiarity with the disease. This is the first time that we've seen Ebola in West Africa. So it kind of caught everybody by surprise. And the second is availability of resources to contain, prevent and treat the people that are sick.
This is important. When we hear that the case fatality rate for Ebola has been 60 to 65% across history, well, that sounds like a fact about Ebola. But it's also a fact about healthcare systems and about how well they treat the symptoms.
And yet, if you catch Ebola, it is still quite likely to kill you.
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Chapters
5 chapters
1
What is the main topic discussed in this episode?
0:00–12:33
2
What does 'up to 90% fatal' actually mean for Ebola case fatality rates?
12:33–15:59
3
Which historical Ebola outbreaks produced the highest and average fatality rates?
15:59–20:25
4
What factors explain variation in Ebola mortality between countries?
20:25–27:03
5
How many Ebola cases might be unreported and what did WHO really say about 12,000 cases?
27:03–28:01