CDC on Tuesday released a report outlining the best- and worst- case scenarios for the Ebola outbreak that has already infected at least 5,800 people in West Africa.
The report includes only projections for Liberia and Sierra Leone and takes into account the fact that many cases have been unreported. The latest data from the World Health Organization show 5,843 confirmed cases of Ebola in Guinea, Liberia, and Sierra Leone and 2,803 deaths. CDC says the actual number of cases could be 2.5 times the number reported.
Our timelineHow the deadliest outbreak in history spread
The CDC report does not include projections for Guinea, where the number of cases has fluctuated in ways that cannot be effectively modeled.
According to the CDC:
The best-case scenario. This model assumes that the dead are buried safely and that 70% of patients are treated in areas that reduce the risk of transmission. In this situation, CDC predicts that the epidemic would be "almost ended" in the two countries by Jan. 20. Currently, about 40% of patients in Sierra Leone and 18% of patients in Liberia receive care in such settings.
The worst-case scenario. This model assumes that trends will continue without a massive and immediate increase in the response to the outbreak and without changes in community behavior. In this situation, CDC predicts that 21,000 people would be infected in the two countries by Sept. 30 and 1.4 million would be infected by Jan. 20.
"My gut feeling is, the actions we're taking now are going to make that worst-case scenario not come to pass," says CDC Director Thomas Frieden, highlighting U.S. commitments to help contain the outbreak. He says the CDC does not take into account steps taken or planned by the international community since August. "We anticipate that these actions will slow the spread of the epidemic," he says.
However, Frieden says, "it’s important to understand that it could happen."
The CDC estimates are based on a tool developed to update outbreak projections as the situation evolves and help planned containment efforts.
Meanwhile, WHO—which last month estimated that the outbreak could reach 20,000 cases before being contained—published its own revised estimates on Monday.
Now, the global health agency projects 20,000 cases by Nov. 2 if containment efforts do not improve. Although the estimate is far smaller than the new CDC projection, the agency says that many cases are unreported and that—without effective aid—the three most-affected nations could soon have thousands of new cases and deaths each week.
The WHO report, published in NEJM, finds that the fatality rate of the current outbreak is 71%. However, the scope of the outbreak is not a result of the virulence of the outbreak, but rather it appears "likely to be a result of the combination of dysfunctional health systems, international indifference, high population mobility, local customs, densely populated capitals, and lack of trust in authorities after years of armed conflict," according to the report.
WHO for the first time has raised the possibility that Ebola could become endemic in West Africa, meaning that it becomes a constant presence in the area. CDC has not addressed the possibility, but it is a scenario that health experts have feared since the outbreak began, according to the New York Times.
In another WHO report, the organization said that 337 health workers have contracted Ebola while caring for infected patients, and 181 of those workers have died (Grady, New York Times, 9/23; Sun, Washington Post, 9/23; Smith-Spark/Falco, CNN, 9/23).