Ebola Death Rate Much Higher Than Press Reports Suggest

Estimating the case fatality rate - death rate - in the current outbreak of Ebola in west Africa depends critically on knowing the length of time between cases being reported and deaths being reported by WHO. Press reports quoting CFRs as low as around 55 per cent make the mistake of assuming this time lag is essentially zero.

Click to enlarge. Estimating the case fatality rate – death rate – in the current outbreak of Ebola in West Africa depends critically on knowing the length of time between cases being reported and deaths being reported by WHO. Press reports quoting CFRs as low as around 55 per cent make the mistake of assuming this time lag is essentially zero. A time lag of three weeks is consistent with a much higher death rate of 90 per cent.

Ebola in West Africa is almost certainly far more deadly than press reports indicate.

Simplistic analysis of the regular updates issued by the World Health Organisation (WHO) has led many reputable news outlets to underestimate the true death rate of this killer disease while the WHO itself avoids issuing an ongoing analysis of what it calls the case fatality rate.

For example: a BBC report updated yesterday states “About 54% of those infected in the current outbreak have died”; a Guardian report published yesterday states the “mortality rate” is “running at about 55% to 60%”; while the New York Times reported recently that “In the current outbreak, about 60 percent of the cases have been fatal”.

 

This BBC report, stating the Ebola death rate is 54 per cent, is one example of many press reports. Courtesy: BBC

Click to enlarge. This BBC report, stating the Ebola death rate is 54 per cent, is one example of many press reports. Courtesy: BBC

These reports are based on a simple division of the number of deaths by the number of cases that are reported in regular updates by WHO. This would seem to make sense, but the trouble is that the epidemic is growing rapidly and there is a time lag between someone being reported as having the virus and someone being reported as dying from the virus, and the simplistic approach adopted by most of the media ignores this fact. This in turn leads to them calculating a death rate that potentially significantly underestimates the actual case fatality rate of the disease in this outbreak. This is one reason – perhaps the reason – why the press is consistently reporting that the death rate in the West African Ebola outbreak is much lower than the 90 per cent more usually associated with this disease.

So: if much of the press reporting is inaccurate, then what is the actual case fatality rate of the current outbreak of Ebola? WHO updates provide an estimate of the number of cases and deaths in each of three categories – confirmed, probable and suspected – for each of the affected countries, but do not give an estimate for the case fatality rate. In its background material on Ebola, WHO states: “Ebola virus disease (formerly known as Ebola haemorrhagic fever) is a severe, often fatal illness, with a case fatality rate of up to 90%”.

Here is an example of the data released by WHO on a regular basis during the current outbreak. Courtesy: WHO

Click to enlarge. Here is an example of the data released by WHO on a regular basis during the current outbreak. Courtesy: WHO

Calculating the case fatality rate while an epidemic is still growing depends on knowing the average length of time between a case being reported and a death being reported – the time lag. The graph at the top of this page plots the death rate by applying different assumptions for this time lag to WHO data on cases and deaths.

If you assume that this time lag is small – say up to five days – then you can easily arrive at a case fatality rate of between 55 per cent and 60 per cent. But if that time lag is increased to between two and three weeks then the case fatality rate rises into the 80 per cent to 90 per cent range.

Estimating the actual time lag is problematic. We know that Ebola takes between two and 21 days to incubate, according to WHO. We also know that once the disease sets in then it can kill within seven days; for example, American victim Patrick Sawyer, who became ill after boarding a plane on 20 July, died on 25 July.

The actual time lag will be an average varying between cases and probably between countries. It is possible that widespread publicity, medical action and faster detection may have increased the time lag implicit in the WHO data in the last month or so.

Mathematically, the time lag could be anywhere from zero to 28 days; in reality, it is likely to be somewhere mid-way between these two extremes. A highly unscientific split-the-difference approach that assumes a time lag of 14 days between a case being reported and a death being reported would suggest a death rate of around 80 per cent in the current Ebola outbreak.

Whatever actual figure, it is virtually certain press reports that around 55 per cent of victims die are wrong, and significantly underestimate the true figure… The real Ebola death rate is far higher than we are being told.

Sources

BBC News report here.

Guardian story here.

New York Times article here.

WHO Ebola virus updates here.

WHO background on Ebloa here.

See also

WHO Chief Warns Of Ebola Mutation Risk here.

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Comments
2 Responses to “Ebola Death Rate Much Higher Than Press Reports Suggest”
  1. NICURN says:

    Thanks so much for your insight. . . Perhaps part of the problem is that journalists often don’t have enough of a science background to really be able to make sense of the raw data they are presented with (much like many of the rest of us).
    There is one other article at
    http://www.foxnews.com/health/2014/08/06/ebola-mortality-rate-expected-to-rise-as-outbreak-runs-its-deadly-course/
    which quotes a virologist who expressed a very similar sentiment. It would seem that this is not a kinder and gentler version of Ebola Zaire after all.

  2. NICURN says:

    At
    http://www.foxnews.com/health/2014/08/06/ebola-mortality-rate-expected-to-rise-as-outbreak-runs-its-deadly-course/
    One commenter suggested a way to get a rough guestimate the true death rate. . .Find the total number of cases of infection reported earlier (your article would suggest about 14 days) and divide by the current number of deaths, thereby accounting for the lag. Starting with 1,013 deaths (8/11: WHO ebola deaths through Aug 9) and dividing by 1,323 cases (7/29: WHO ebola cases through 7/27) you get an estimated 76% death rate assuming a 13 day lag. I realize that there are all sorts of slush factors I haven’t accounted for, but it backs up what you’ve said—that the death rate may be significantly higher than what journalists have understood it to be.

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