Case Fatality Rate for ebolavirus

A case fatality rate (CFR) or case fatality risk is a property of an infectious disease in a particular population which states the risk of fatality due to the disease per case. The first thing to note is that it can't be reliably be calculated for an ongoing epidemic by dividing the reported number of deaths due to a disease by the reported number of cases. There are a number of reasons why this will be a poor estimate: Firstly, this doesn't take into account infections that have yet to run their course (ending in recovery or death). If many new cases are being reported, then this will under estimate the CFR. A more reliable estimate can be made if the number of recoveries is also being reported. Secondly, the estimate will be poor if there is a bias in reporting or diagnosis towards severe cases of the disease. This is the case with MERS-CoV in the Middle East where there are many instances of mild or asymptomatic cases (discovered through contact tracing) but most primary cases are only tested if the patients are hospitalized. This will overestimate the CFR. With ebolavirus, bias may occur if patients are being looked after at home and only being hospitalized or recorded if the disease becomes very severe or if the patient dies. 

Considering the numbers from the latest WHO disease outbreak news:

http://www.who.int/csr/don/2014_08_06_ebola/en/

Total number of suspected and confirmed deaths and cases up to the 4th August 2014.

Country	Deaths	Cases	Ratio
Guinea	363	495	0.73
Liberia	282	516	0.55
Sierra Leone	286	691	0.41

This table shows a striking difference in the ratio of reported deaths to reported cases amongst the 3 countries most affected. This might be the result of differences in health care amongst the countries (although there is no specific treatment for ebola virus disease (EVD), general care from an early stage can improve the chances of recovery). However, the other thing to note is that these ratios are in inverse order to the size of the epidemic in number of cases and Liberia and Sierra Leone have had a much higher rate of reporting new cases than Guinea over recent weeks. 

Another way to look at the numbers is to consider only laboratory confirmed cases. This might seem like a good idea as this would remove suspected cases that were caused by other diseases (malaria, Lassa virus, etc) which may be less fatal than EVD. Here is the numbers for only confirmed cases from the same WHO DON:

Total number of EVD confirmed deaths and cases up to the 4th August 2014.

Country	Deaths	Cases	Ratio
Guinea	228	351	0.65
Liberia	143	128	0.90
Sierra Leone	247	576	0.43

This is much more confusing with Guinea dropping to 0.65 and Liberia going up to 0.9. It is difficult to know what is happening here but counties may differ in how they decide which cases to test and it is possible that Liberia is focusing on testing fatal cases or having a greater proportion of cases being reported post-mortem. Plotting these ratios over the last month, see below, shows that Liberia has consistently had a greater proportion of deaths in their confirmed cases suggesting this sort of bias. This ratio has then shot up to 0.9 in the last couple of reports. Given Guinea's relatively slower rate of reporting of new cases, I suspect that the 0.7-0.75 ratio may be a better reflection of the underlying CFR. 

However, until a careful epidemiological analysis is done, one that includes only those cases for which the outcome is known, reports of case fatality rates are unreliable. I would also be very sceptical about claims that this virus is different from previous Zaire-lineage ebolavirus or that it is changing in character over time.

Update 19-Aug-2014

I came across this New York Time article which quotes Medecins Sans Frontieres as reporting "only 61 of the 337 Ebola patients treated at its tent-camp treatment center in nearby Kailahun have survived".  This would estimate a CFR at 0.82 (binomial 95% confidence intervals of 0.77, 0.86). However, this still doesn't say whether this 337 patients number includes patients still being treated. This is in Sierra Leone. A report on the Medecins Sans Frontieres is reporting some different figures. For Sierra Leone it states this:

"In total, MSF treatment centers have admitted 294 patients, of whom 191 were confirmed to have Ebola. Of those, 47 people have recovered and returned home."

Which gives a CFR of 0.84 (0.79, 0.88). However, this suggests this is across all treatment centers and the number is lower than the NYT report about just the one center. 

The MSF report states that "there are currently 50 patients in the center", referring to the Kailahun centre. So perhaps these should be removed from the figures? This would give numbers (taking the NYTs figures) of 61 recovered, 50 still being treated implying 226 fatalities. This would give a CFR of 0.79 (0.74, 0.83). 

For Guinea there is this:

"Since March, MSF treatment centers in Guékédou have admitted 366 patients, of whom 169 were confirmed to have Ebola. Forty-seven patients have recovered and returned home"

The article says that there are 11 patients currently being treated in Guékédou so that would imply 308 fatalities out of 355 resolved cases with an CFR of 0.87 (0.83, 0.90).