It’s only a matter of time, they add, before the virus could start spreading to other places, including previously unaffected countries in Africa and developed nations like the United Kingdom -- and the U.S., according to a paper published Sept. 2 in the journal PLOS Currents Outbreaks.

There’s a roughly 25 percent chance Ebola will be detected in the United Kingdom– and as much as an 18 percent chance it will turn up in the U.S. – by the end of September, the analysis of global mobility and epidemic patterns shows. The new paper includes the top 16 countries where Ebola is most likely to spread.

Though concerning, a spread to Western nations is not the biggest threat. At most, there would be a cluster of a few cases imported to the U.S., probably through air travel.

“In the U.S., where one has good infection control practices, even if a case were to get in, it’s likely the infection could be contained,” said Dr. Betz Halloran, a biostatistician and member in the Fred Hutch Public Health Sciences and Vaccine and Infectious Disease divisions.

The chief concern is that the rising cases are rapidly outpacing international efforts to contain the spread, said Dr. Alessandro Vespignani, the Northeastern University physics professor who developed the Global Epidemic and Mobility Model to assess outbreaks.

“We are at a crucial point,” Vespiginani said. “If the number of cases increases and we are not able to start taming the epidemic, then it will be too late. And then it requires an effort that will be impossible to bring on the ground.”

As of Aug. 31, the World Health Organization said there were 3,685 probable, confirmed or suspected Ebola cases in Guinea, Liberia and Sierra Leone, and 1,841 deaths.

But latest counts Monday from the Centers for Disease Control and Prevention, which include WHO and Ministry of Health reports, put the total at 4,061 cases and 2,107 deaths.

There’s no proven vaccine or treatment for the infection other than supportive care. Experimental drugs and blood transfusions have been used in some cases and a potential vaccine is being rushed into production.

Top health officials from WHO and the CDC and the medical charity Doctors Without Borders each outlined the dire situation in separate news conferences last week.

“There is a window of opportunity to tamp this down, but that window is closing,” the CDC’s director, Dr. Tom Frieden, told reporters. “We need action now to scale up the response.”

‘I hope to be wrong’

WHO officials have predicted as many as 20,000 cases of Ebola and laid out a “road map” for the outbreak response that calls for stopping the outbreak within six to nine months. But that’s only if a “massive” global response is implemented.

The scenario modeled in the new paper suggests that the actual number of cases could far exceed the WHO estimate – and far sooner. Vespignani said he and his colleagues are calibrating the model every couple of weeks to see whether there’s any change. So far, the answer is no.

“The data points are still aligned with the worst-case scenario,” Vespignani said. “It’s a bad feeling. I hope to be wrong.”

That’s a sentiment echoed by Longini, who said that he and other disease modelers are dismayed by what they see.

“There’s nothing to be optimistic about,” he said. “It’s frustrating. It feels like there should be a more concentrated international effort to help these countries.”

Dr. David Nabarro, the United Nations coordinator for Ebola response, said it would take at least $600 million and an international response three to four times the current level.

There’s a huge need for doctors, nurses and other health care workers, as well as protective gear for health workers and basic supplies such as gloves and chlorine.

One problem is that several airlines have shuttered routes to the Ebola-affected countries, hoping to stop the spread of the disease. Closing off 80 percent of traffic would only delay the course of the epidemic by three or four weeks, according to the new paper, but it would also cripple efforts to send supplies and personnel to help.

“The results show that reducing airline traffic partially will not prevent spread of Ebola, but merely delay it,” Halloran said. “Given the high social and economic cost of such disruption, it does not seem to be a good intervention to recommend.”

Halloran, Longini and other researchers at Fred Hutch are among experts advising the U.S. component of the international Ebola response. In addition to the work modeling the potential spread of the epidemic, the researchers are part of the Models of Infectious Disease Study, or MIDAS, network.

The modeling is invaluable, said Dr. Martin Meltzer, who leads the CDC’s health economics and modeling unit. The data available so far in the Ebola outbreak is scant, far more rudimentary than the experts are accustomed to using.

But even with the thin data, the models are raising alarms about the runaway spread of the outbreak and highlighting the biggest need – to find ways to stop the spread of the virus on the ground.

“These models are mostly useful in getting people to pay attention,” he said, noting that he’s been modeling disease epidemics for three decades. “This is one of the few times, if not the only time, when I can say I wish I was wrong.”

Next three weeks are crucial

Ebola is a viral hemorrhagic fever, one of several severe, often fatal similar diseases that affect humans and non-human primates such as monkeys, gorillas and chimpanzees.

It’s not spread through air, like a cold or flu, but through direct contact with bodily secretions. In the current outbreak, which is the largest in history and the first in West Africa, health workers and family members who’ve been preparing bodies for funerals appear to be responsible for the greatest spread of the virus.

There’s great fear and stigma in villages where Ebola has run rampant, health officials said. There’s little understanding of how the virus spreads and a belief that doctors or other Westerners are actually bringing the disease with them. Some infected people have fled or hidden to avoid care, further exacerbating the spread.

The next three weeks will be crucial to determining whether the Ebola outbreak is tamed or rages out of control, the experts agreed.

“I do think the disease is not that infectious, easily identifiable and it is containable with a reasonable amount of effort,” Longini said. “I don’t think that it’s totally hopeless.”

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