WHO, CDC sketch varying Ebola projections


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Editor's note: This story was revised on Sep 23, 2014, to clarify that the CDC did not report a range of projected case totals, but rather projected distinct numbers of reported cases and total cases.

In an effort to gauge how bad West Africa's Ebola epidemic could get without immediate and massive intervention, two agencies released estimates, one suggesting up to 20,000 infections by early November and another projecting as many as 1.4 million cases in Liberia and Sierra Leone by the middle of January, without additional interventions.

Experts who put together the estimates used different types of data and produced their findings for different reasons. The more conservative estimate comes from the World Health Organization (WHO) Ebola Response Team, which used epidemiologic and clinical data from a subset of patients in the first 9 months of the outbreak to provide a snapshot of the outbreak.

The second estimate is from a team at the US Centers for Disease Control and Prevention (CDC) who used a modeling tool to help outbreak responders gauge the possible effectiveness of different interventions and timetables.

The WHO team published its findings today in an early-release report in the New England Journal of Medicine (NEJM), and the CDC group's report appears in an early edition of Morbidity and Morality Weekly Report (MMWR).

First comprehensive epidemiologic report

In the first report, researchers from the WHO and Imperial College London gathered clinical and demographic data from Ebola virus disease (EVD) patients who received care in Guinea, Liberia, Nigeria, and Sierra Leone, fleshing out the information with informal case reports, lab reports, and burial records. They identified clinical and transmission patterns, risk factors, and disease outcomes.

Though anecdotal reports have suggested that the burden of EVD might be higher in women because they are more likely to care for the sick, the team learned that there was no significant difference in incidence for men and women in any of the countries. Christopher Dye, PhD, study coauthor and director of strategy for the WHO, said in a statement, "There may be differences in some communities, but when we actually looked at all the data combined, we saw it was really almost split 50-50."

The overall case-fatality rate (CFR) they found for the three hardest-hit countries was 70.8%, higher than the 50% that some sources have suggested. However, the rate was lower (64.3%) in hospitalized patients, which the WHO team said shows that getting timely supportive care can make a difference.

Their reproductive-number estimates of disease transmission have fluctuated some over the course of the epidemic so far, especially in Guinea and Sierra Leone, but the WHO authors wrote that the growing number of cases in August and early September shows a still-expanding epidemic. Given the increased rate, they estimated that with no stepped-up control efforts, cases would soar past 20,000 by Nov 2.

Though the scope of West Africa's epidemic is unprecedented, the WHO team said its analysis suggests that the clinical course and virus transmissibility are similar to patterns seen with earlier EVD outbreaks. Also, they noted that the incubation period, illness duration, CFR, and reproductive numbers are all within ranges found for other Ebola events.

The extremely large scale of the region's outbreak, the authors said, is probably the result of certain characteristics of the population—such as brisk cross-border activity in the main disease hot spot and already-fragile health systems—and of inadequate control efforts. They estimated that cutting transmission by a little more than half could eliminate the disease, as would a new vaccine, if the coverage level exceeded 50%.

They concluded that the outlook for the region is "bleak," and without quick improvements in control measures such as contact tracing and adequate case isolation, the three countries will soon be reporting thousands of cases and deaths each week, projections that are similar to the CDC's.

Experts warn of endemic EVD in West Africa

In an NEJM editorial that accompanied the WHO team's report, two London-based infectious disease experts wrote that the epidemic, with its profound effect on the countries' health systems, could help trigger a breakdown in civil society as people lose faith in their governments. The authors are Jeremy Farrar, MD, PhD, director of the Wellcome Trust, and Peter Piot, MD, PhD, director of the London School of Hygiene and Tropical Medicine, who is known for helping to identify the Ebola virus in 1976.

They called today's NEJM report the first comprehensive analysis of surveillance so far in West Africa's epidemic. Acknowledging the difficulty of data collection in such difficult conditions, they said the team provides a convincing case that the outbreak is still expanding.

"Without a more effective, all-out effort, Ebola could become endemic in West Africa, which could, in turn, become a reservoir for the virus's spread to other parts of Africa and beyond," Farrar and Piot wrote, adding that a highly inadequate and late response has contributed to that worrisome prospect.

The outbreak offers three lessons for addressing rapidly evolving public health emergencies, they said: Factors such as poor health infrastructure and local customs can transform a limited outbreak into a massive one; classic outbreak control efforts aren't enough for an event of this size; and developing diagnostic tests, drugs, and vaccines should be prioritized between epidemic periods. "We believe that in this epidemic, we are reaching the limit of what classic containment can achieve," they wrote.

Modeling tool shows intervention benefits, cost of inaction

In the MMWR report, the projection of possible EVD cases for Liberia and Sierra Leone comes from a modeling tool constructed by CDC experts. Based on previous EVD models and data from August in the countries, the tool is designed to help outbreak response planners make decisions about response steps such as isolation and safer burials; it will be freely available in a Microsoft Excel spreadsheet. CDC officials said multiple separate waves of EVD activity in Guinea made it impossible to include that country in the modeling tool.

By the end of September, cases could reach a range between 8,000 and 21,000, according to the CDC model. It calculates that by the middle of January, barring more interventions or changes in community behavior, the total in the two countries could reach about 550,000 reported cases or 1.4 million total cases, including reported and unreported ones. The higher number assumes that there are 2.5 unreported cases for each reported case.

The CDC team included several caveats with its estimate, including that the estimate reflects only what was known in August and doesn't consider ongoing US response efforts.

Modeling calculations suggest that cases in Liberia are doubling every 15 to 20 days and that in Guinea and Sierra Leone, cases are doubling every 30 to 40 days, the CDC said.

At a media briefing today, CDC Director Tom Frieden, MD, MPH, downplayed the estimate's use as a projection, because it is based on already outdated numbers and doesn't consider response efforts, which he said have started to improve over the past few weeks. Rather, the model and estimates are designed to illustrate that the interventions are likely to have an important impact and send the message that the human cost of delaying the response would be very high.

"The bottom line is that the model shows a surge now can break the back of the epidemic," he said.

Gayle Smith, special assistant to President Barack Obama and senior director with the National Security Council, told reporters that the data suggest how to bend the epidemic curve but don't take into account the response surge over the past few weeks. The United States and other key groups that are leading the response need to keep the pressure on the international community to do more, and she added that with 700 treatment unit beds being added by the United Kingdom and significant contributions from Asian countries, the African Union, and other groups, "We are starting to see a significant (response) surge."

Model show 70% isolation 'sweet spot'

The CDC team that published the modeling report said stopping the epidemic requires isolating up to 70% of patients in treatment centers or other settings that reduce transmission, assuming that burials are handled safely. Frieden said one encouraging message from the modeling report is that once isolation levels reach that target, cases plummet rapidly—almost as fast as they rose.

Frieden said that because treatment centers are overwhelmed, outbreak responders are looking at other ways to isolate sick people, such as locations in the community where they can receive food, rehydration therapy, and pain medicine.

WHO Ebola Response Team. Ebola virus disease in West Africa—the first 9 months of the epidemic and forward projections. N Engl J Med 2014 Sep 23 [Full text]

Farrar JJ, Piot P. The Ebola emergency—immediate action, ongoing strategy, editorial. N Engl J Med 2014 Sep 23 [Full text]

Meltzer MI, Atkins CY, Santibanez S, et al. Estimating the future number of cases in the Ebola epidemic—Liberia and Sierra Leone 2014-2015. MMWR 2014 Sep 23;63(1-14): [Full text]

See also:

Sep 22 WHO statement

Sep 23 CDC press release

Sep 23 CDC Q and A on Ebola modeling tool

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