70 percent Ebola death rate? Here’s how they calculate it

Several viewers wrote in this week, questioning our math in an Ebola update that aired on Tuesday’s PBS NewsHour.

We reported the World Health Organization’s latest figures: 8,900 people had contracted Ebola and 4,450 people had died, and that the WHO is now saying Ebola has a 70 percent fatality rate.

One viewer wrote: “4,450 divided by 8,900 is only 50 percent. What were you trying to say? The error seems obvious to me.”

We contacted the WHO to explain its numbers, and it turns out it isn’t as simple as dividing the latest total cases by the number of deaths.

Our contacts at the WHO pointed us to an article published in the New England Journal of Medicine on Sept. 23, called, “Ebola Virus Disease in West Africa – The First 9 Months of the Epidemic and Forward Projections,” as the source of the 70 percent fatality rate.

Using data supplied to the WHO, the researchers came up with the 70 percent estimate using a subset of the number of Ebola cases as of Sept. 14 in four main Ebola-affected countries — Guinea, Liberia, Nigeria and Sierra Leone — with a known clinical outcome.

The “known clinical outcome” is key. When they calculated the estimate from the ratio of all reported deaths to all reported cases, the death rate was lower than that of past Ebola outbreaks and varied widely among the West African countries hardest hit by the virus.

When they used only 46 percent of the cases that had a definitive recorded clinical outcome, they got a higher estimate — 70.8 percent — that was consistent among Guinea, Liberia and Sierra Leone. The fatality rate in Nigeria, which was based on only 11 cases, was much lower at 45.5 percent. (Watch NewsHour special correspondent Fred de Sam Lazaro’s report on why Nigeria is succeeding in containing Ebola.)

The fatality rate for patients treated at hospitals was slightly lower, as one would expect, at 64.3 percent and was consistent among the three countries as well.

Most Ebola sufferers (60.8 percent) were between ages 15 and 44, and there wasn’t much of difference between males and females getting the disease, the study also showed.

The journal authors ended with: “Without drastic improvements in control measures, the numbers of cases of and deaths from [Ebola] are expected to continue increasing from hundreds to thousands per week in the coming months” — another finding the WHO has started citing.

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