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FiveThirtyEight

As the worst Ebola outbreak in history spreads through West Africa, media reports have repeatedly cited health officials who say the virus kills up to 90 percent of people it infects. CNN has skipped the “up to” in several stories this week, reporting that typically 90 percent of people infected with Ebola die from the infection — and that the 60 percent mortality rate during the current outbreak reflects success from early treatment.

Behind these numbers, though, lies considerable uncertainty: about the typical death rate for Ebola, about the death rate for the current outbreak and about what that rate will end up being when the infection is halted.

The World Health Organization, the United Nations’ health arm, has compiled the number of cases and deaths from two dozen prior outbreaks, going back to the first outbreak, in 1976. Technically, the mortality rate per outbreak has ranged from 0 percent to 100 percent: Once in the Ivory Coast two decades ago, one person was infected and that person survived; and three times, a single infected person died.

Ignore those single cases, and the mortality rate has ranged from 25 percent (among 149 infected people in Uganda in 2007) to 90 percent (among 143 infected people in Congo in 2003). Overall, of 2,387 people diagnosed with Ebola in prior outbreaks, 1,590 people died — almost exactly two out of three.

In the four months of the current outbreak in Sierra Leone, Liberia and Guinea (and most recently Nigeria), the death rate has been lower: 56 percent through last Wednesday, the latest available numbers from the WHO.

Some news articles and doctors have attributed the lower death rate to catching cases early and providing infected people with treatment. There is no cure for Ebola, but hydration, electrolytes and bed rest can help manage symptoms, especially diarrhea. “The mortality rate in some outbreaks can be as high as 90 percent, but in this outbreak, it is currently around 60 percent, indicating that some of our early treatment efforts may be having an impact,” Stephan Monroe, deputy director of the U.S. Centers for Disease Control and Prevention’s National Center for Emerging Zoonotic and Infectious Diseases, said in a press briefing on Monday.

But there are several reasons to interpret the reported death toll as very preliminary; some reasons mean it might rise from here, while others mean it might fall. “There are things that could cause the overall number to be underestimated or overestimated,” Gregory Härtl, a spokesman for the WHO, said in a telephone interview on Tuesday. “That’s why you really have to take these numbers with a grain of salt.”

First, here are reasons why the true mortality rate of the current outbreak might be higher than current figures suggest:

  • The outbreak isn’t over yet. The disease often takes weeks to progress once there are symptoms. The WHO added more than 300 people to its count of cases in the 11 days through July 23. Those people aren’t out of the woods. In the latest numbers, just 12 deaths but 108 new cases were added over three days, for a mortality rate of one in nine. Before that update, the mortality rate had been between 60 percent and 63 percent for each of the last eight updates, every few days over three weeks. “I’d still expect about another 200 to 250 people to die from this even if not another single case occurs,” Benjamin Neuman, a virologist at the University of Reading’s School of Biological Sciences in the U.K., said in a telephone interview on Tuesday. “That will play itself out over the next month. There will be a considerable amount more death, even if the health response is perfect.”
  • Some false positives are likely, and they artificially inflate case totals, suppressing the mortality rate. False negatives also are possible, but less likely. Confirming a case requires multiple tests, and each test creates risk of infection for medical workers, so better to err on the side of caution and quarantine and treat everyone who might be infected. Nearly one-third of the cases counted by the WHO aren’t confirmed. And less than one-quarter of the people whose diagnosis is labeled as suspect have died.

There are also reasons why the true mortality rate might be lower than reported:

  • People who exhibit symptoms common to both Ebola and to many other infectious diseases and then recover are less likely to be tested than are people who are severely ill.
  • And people who die from Ebola without being diagnosed likely will eventually be identified as victims of the virus, said the WHO’s Härtl, because traditional burial practices in the affected countries often lead to infections of people involved in the burials.

Together, those two factors mean the cases could be more undercounted than deaths are. Russian medical official Vladimir Nikiforov made a similar point when arguing in April that the mortality rate in West Africa is an overestimate.

Other factors could go either way, including the mixed accuracy record of labs involved in testing. “There have been some issues with this outbreak with labs having incorrect results,” Härtl said.

When this outbreak is done, it could include the majority of all Ebola cases in history, and perhaps help rewrite the conventional wisdom on the disease. But several factors will complicate the learning process. First, the strain is similar but not identical to prior strains: It is 97 percent identical, genetically, to a strain that struck Congo and Gabon in the past. Second, the same factors that call into question current mortality rates apply to prior outbreaks, meaning any differences could be the result of measurement rather than actual differences in the disease. And third, conducting research on the disease is highly dangerous and restricted to just a few dozen labs worldwide with Biosafety Level 4 clearance.

“It slows down research but keeps people safe,” Neuman said of the lab restrictions. “You have to balance those things.”

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