The Ebola virus that is causing the epidemic in West Africa is famous lethal. In previous outbreaks, he killed as many as 0% of those it infects. That's why the latest figures from the World Health Organization (WHO) "Status Report" look like they could be a rare glimmer of good news. Although the infection rate is increasing at an alarming rate, the report states that the mortality rate was 53% overall, ranging from 64% in Guinea to only 39% in Sierra Leone
But there is a snag .: the apparent low proportion of deaths probably depends more on how the health authorities calculate that the number on the deadliness of the virus or the quality of care patients receive. Indeed, the dramatic increase in cases in recent weeks is one of the main reasons why the rate of reported deaths appears to be artificially low.
There are several ways to calculate what officials call the "case fatality rate", or CFR, a disease outbreak. One of the simplest is to divide the number of deaths by the total number of cases. That is what the WHO in its recent CFR calculations.
But this method does not take into account that many living recently-diagnosed patients and poorly will not survive. So it underestimates the death rate. And this effect is exaggerated when an epidemic is growing rapidly. The calculation also lack of patients who were confirmed as cases of Ebola, but then left the hospital before being discharged, said Andrew Rambaut, an evolutionary biologist who studies infectious diseases at the University of Edinburgh in the UK -United. Many of these patients later died, but are not counted in the death statistics.
Another way to calculate the rate is to ignore current patients and count only patients who officially recovered and released from treatment or is known is dead. These figures seem to paint a picture more thinking. According to the update 7 September the Sierra Leone Ministry of Health and Sanitation, 268 patients were treated and released, and 426 confirmed cases of Ebola died. These figures suggest a fatality rate of 61%. But that's not quite true either, says Marc Lipsitch, an epidemiologist at the Harvard School of Public Health in Boston: Survivors may have long hospital stays means that patients who die. This would lead to a CFR that is artificially high.
A more accurate way to calculate the rate is to compare the outcomes of patients who were infected at the same time and wait long enough until all have either recovered or died. Rambaut notes that there were 23 survivors among the 77 patients included in a recent paper looking at the evolution of the virus. It is a CFR of 70%.
Christopher Dye, director of strategy for the WHO, said the organization is heading this method works and compiling data for each patient registered as a case. "We need reliable estimates," says Dye. "We want to know if CFR is different in this epidemic precedents in Central Africa [and] if different approaches to patient care in the current epidemic lead to different results."
Even this method is imperfect. in almost every home, the cases are missed because the patient never seeks care at a health facility, and therefore are not recorded in the statistics. These missed cases can potentially distort the CFR in both meaning, Lipsitch note. If many cases are relatively mild in which people recover without ever seeing a doctor then count on health care records overestimate the mortality rate. (This was the case for pandemic influenza H1N1 in Mexico, and experts suspect it is the case for MERS virus as well.) a Benin Ebola cases is less likely to go unnoticed in a Benin cases of influenza, Lipsitch said, but given the general lack of care health in the region, there could be a significant number of detected survivors.
on the other hand, researchers already know that many victims of Ebola never arrived at the hospital and died at home (often infecting family and other caregivers). This means that their deaths are not counted, the reduction of CFR.
Exactly how many unregistered Ebola deaths occurred will never be known. Health authorities keep track of suspect and probable cases, many people who died before they could be tested. Whether to include these numbers in the calculation of the CFR is another source of potential bias. And there are different models of tests in different regions: Some places have more tests on post-mortem, for example. "How to balance these prejudices is always the big question," says Lipsitch.
"We are not naive about the difficulties of CFR estimate," Dye wrote in an email. "I'm not yet ready to believe ... that CFR is much higher than in Guinea Sierra Leone. So say the data, taken at face value, but we need to exclude any possibility of ascertainment bias before believing this to be the truth "
* Ebola files :. given the current Ebola outbreak unprecedented in terms of the number of people killed and the rapid geographic spread, science and science Translational Medicine made a collection 'research articles and news on the viral disease available for researchers and the general public
* Correction, September 8, 1:53 p.m. :. the mortality rates in Guinea and Sierra Leone have been corrected.
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