Ebola fatality rate is drastically underestimated by health officials due to rapidly accelerating spread

(NaturalNews) The fatality rate for the current Ebola outbreak in West Africa is being drastically underestimated, public health experts have warned.

For the past six months, Ebola has been spreading rapidly across several West African nations. Although prior Ebola outbreaks have had death rates of as high as 90 percent, the numbers released in the latest "Situation Report" from the World Health Organization (WHO) are significantly lower. The report notes a 53 percent overall death rate, varying from a low of 39 percent in Sierra Leone to a high of 64 percent in Guinea.

Unfortunately, this low case fatality rate (CFR) probably stems more from the difficulty of accurately counting Ebola cases and fatalities during an epidemic than from improvements in treatment or a less deadly strain of the virus, experts say.

"We are not naive about the difficulties of estimating CFR," said Christopher Dye, director of strategy for WHO. "I'm not yet ready to believe... that CFR is much higher in Guinea than Sierra Leone. This is what the data say, taken at face value, but we need to exclude all possibility of ascertainment bias before believing this to be the truth."

Rapid spread obscures true death rate

CFR is defined as the proportion of cases of the disease that lead to death. The most straightforward way to calculate it is to simply take the number of recorded fatalities and divide it by the number of reported cases to date. The problem is that, because many of the newest cases will eventually lead to deaths, this method delivers an artificially low CFR. This is especially true during periods when a disease is spreading rapidly, as with the current Ebola epidemic -- new cases are appearing rapidly, but the deaths from those cases may still be several weeks off.

This method of calculating CFR also fails to count cases in which a patient goes to a hospital and is confirmed as infected with Ebola but then goes home and dies there, said evolutionary biologist Andrew Rambaut of the University of Edinburgh.

Other methods of calculating CFR deliver drastically different numbers. For example, counting only patients who are known to have either died or recovered (in other words, excluding new cases or people who left hospitals before the disease had run its course) yields a 61 percent fatality rate for Sierra Leone, rather than 39 percent. But even this method has its problems, says epidemiologist Marc Lipsitch of Harvard School of Public Health. Because survivors may tend to stay hospitalized longer than patients who die, this method makes the CFR artificially high by failing to count some patients who actually go on to recover.

Flaws in even the best method

The method favored by WHO involves taking a group of patients who were all infected around the same time, then waiting until all of them have either died or recovered. Although the WHO is still compiling this data for CFR, Rambaut noted that a recent paper examining the evolution of the virus tracked 77 patients infected around the same time and found that only 23 survived. That would translate to a CFR of 70 percent -- much higher than the 53 percent currently reported by the WHO.

Yet even the best method is only as good as its data, and experts admit that many patients in the current epidemic are both dying and recovering without ever being counted. Other problems include different techniques used to test for the virus in different countries and different rates of testing corpses to see if they died from Ebola. Any of these differences could artificially raise or lower CFR calculations.

"How these biases balance is always the big question," Lipsitch said.

Learn all these details and more at the FREE online Pandemic Preparedness course at www.BioDefense.com

Sources:

http://news.sciencemag.org

http://who.int [PDF]

http://science.naturalnews.com