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A Virtual Outbreak Offers Hints Of Ebola's Future

Kenyan health officials take the temperatures of passengers arriving at the Nairobi airport on Thursday. Kenya has no reported cases of Ebola, but it's a transportation hub and so is on alert. (AFP/Getty Images)

While the Ebola outbreak continues to rage in West Africa, it is also unfolding — in a virtual sense — inside the computers of researchers who study the dynamics of epidemics.

Policymakers look to these simulations to get a sense of how the outbreak might spread. They also can use them to run experiments to see which public health measures should take priority.

"I've spent a lot of time doing computer models of disease transmission, but rarely does it involve something in Africa. Africa is often overlooked," says Bryan Lewis, a computational epidemiologist at Virginia Tech.

So when a defense agency called him a few weeks ago and asked him to model the Ebola outbreak, he was excited by the challenge.

Lewis started plugging data into his computer. He uses the official numbers of how many people have died or gotten infected, even though those are probably underestimates. And he says health officials really don't have a handle yet on other important stuff that's going on — like how many infected people stay at home versus go to a hospital, or how burial practices spread infection.

"Some of those factors are the ones that are hard to measure," he says. "You've got to choose how much of this complexity you care to explicitly represent."

What's more, they can't assume this will play out like past Ebola outbreaks — those hit much smaller populations in more isolated, rural areas.

Despite all this uncertainty, Lewis says his models have been able to predict the course of the epidemic so far.

"At the moment, these models — at least for Sierra Leone and Liberia — we aren't putting in any mitigating factors. We're just letting these things run unthrottled," Lewis says. "And they've just been surging up. And they've been, unfortunately, accurate in the last couple of weeks in terms of the number of cases coming out."

He says if you just kept this simulation going on and on, it shows Ebola spreading across the continent. But this scenario he's constructed doesn't include all the public health measures starting to ramp up now.

"We know in the real world there are efforts being directed out there, there are resources being allocated," says Lewis. "Until we understand that better and can incorporate that into the model, I don't think it's very useful to speculate out past a week or two."

Some computer simulations focus on the risk of Ebola spreading to other countries. Alessandro Vespignani, at Northeastern University, creates those models, using information about air travel and other kinds of transportation.

His work suggests that Ebola could find its way to African nations like Ghana, Gambia, and Senegal. "There is a tangible risk of spreading in the region to other countries," says Vespignani, "probably in the ballpark of 20 to 30 percent in the next few weeks."

He notes poor countries might have trouble keeping an imported case from spreading. And the larger this outbreak gets, the harder it will be to contain.

So while his model currently suggests that the risk of Ebola reaching the U.S. or Europe in the next six weeks or so is very small — just a small percentage — that could change if the outbreak in Africa continues to grow.

Vespignani says we need "to extinguish the fire," so that Ebola doesn't really become a threat to the rest of the world in the next months.

Given that all this modeling is as much an art as a science, different groups working on the problem have been comparing notes. They've also been fielding calls from government officials and policymakers.

Martin Meltzer, who heads up the unit at the Centers for Disease Control and Prevention that's been creating computer models of the outbreak, says that people always ask him the same two questions: "How many people are going to die, and when is this going to end?"

He tells them too much is unknown to give any reliable answer.

Mostly, he says, the models just illustrate the need for old, tried-and-true methods for disease control, such as quickly identifying patients and isolating them.

"Modeling won't stop this disease," says Meltzer. "We know how to stop this disease. It's fairly simple and it's a matter of getting the simple activities and practices in action — in place, on the ground."

That's the struggle now, he says. Because while it's easy to change a line of computer code in a simulated epidemic and, say, reduce a transmission rate by 80 percent, it's a lot harder to do that in the real world.

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Transcript

MELISSA BLOCK, HOST:

While the Ebola outbreak continues to rage in West Africa, it is also unfolding virtually inside computers. Researchers are studying the dynamics of the epidemic, and policy makers turn to these experts for predictions of how far Ebola might spread. NPR's Nell Greenfieldboyce explains.

NELL GREENFIELDBOYCE, BYLINE: Bryan Lewis works at Virginia Tech. He describes himself as a computational epidemiologist. He creates mathematical simulations of how infected people spread disease.

(SOUNDBITE OF ARCHIVED RECORDING)

BRYAN LEWIS: And then we run it through different scenarios. And since it's a computer program, we can say, what if we're able to isolate them better once they were identified, or what if we could identify more of them and see how that affects the overall flow of the disease through the population?

GREENFIELDBOYCE: A few weeks ago, a defense agency asked him to model the Ebola outbreak. So he started plugging data into his computer, like the official numbers on how many people have died or gotten infected. Those are probably underestimates, and Lewis says they really don't have a handle on other important stuff that's going on, like how many infected people stay at home versus go to a hospital, or how burial practices spread infection.

(SOUNDBITE OF ARCHIVED RECORDING)

LEWIS: Some of those factors are the ones that are hard to measure.

GREENFIELDBOYCE: Once more, they can't assume this will play out like past Ebola epidemics. Those hit smaller populations in more isolated, rural areas. Despite all this uncertainty, Lewis says his models have been able to predict the course of the epidemic so far.

(SOUNDBITE OF ARCHIVED RECORDING)

LEWIS: At the moment, these models - at least for Sierra Leone and Liberia - we aren't putting any mitigating factors. Like we're just letting this thing run unthrottled and they're just surging up. And they've been, unfortunately, accurate in the last couple of weeks in terms of the number of cases coming out.

GREENFIELDBOYCE: He says if you just kept the simulation going on and on, it shows Ebola spreading across the continent. But this scenario he's constructed doesn't include all the public health measures that are ramping up now.

(SOUNDBITE OF ARCHIVED RECORDING)

LEWIS: We know in the real world there are efforts being directed out there, there are resources being allocated. And until we understand that better and can incorporate that into the model, I don't think it's very useful to speculate out past a week or two.

GREENFIELDBOYCE: Some computer simulations focus on the risk of Ebola spreading to other countries. Alessandro Vespignani at Northeastern University creates those using information on air travel and other kinds of transportation. He says Ebola could find its way to African nations like Ghana, Gambia and Senegal.

(SOUNDBITE OF ARCHIVED RECORDING)

ALESSANDRO VESPIGNANI: There is a tangible risk of spreading in the region to other countries, probably in the ballpark of 20 to 30 percent in the next few weeks.

GREENFIELDBOYCE: He notes poor countries might have trouble keeping an imported case from spreading, and the larger this outbreak gets, the harder it will be to contain. So while his model currently suggests the risks of Ebola reaching the U.S. or Europe in the next six weeks or so is very small - just a few percent - that could change if the outbreak in Africa continues to grow.

(SOUNDBITE OF ARCHIVED RECORDING)

VESPIGNANI: We need really there to extinguish the fire so that then it doesn't really become a threat to the world in the next months.

GREENFIELDBOYCE: The unit at the Centers for Disease Control and Prevention that's been modeling the outbreak is directed by Martin Meltzer. He says government officials always ask him the same two questions.

(SOUNDBITE OF ARCHIVED RECORDING)

MARTIN MELTZER: How many people are going to die and when is this going to end?

GREENFIELDBOYCE: He tells them too much is unknown to give any reliable answer. Mostly, the models just illustrate the need for old tried and true methods for disease control, such as quickly identifying and isolating patients.

(SOUNDBITE OF ARCHIVED RECORDING)

MELTZER: Modeling won't stop this disease. We know how to stop this disease, it's fairly simple, and it's a matter of getting sample activities and practices in action in place on the ground.

GREENFIELDBOYCE: That's the struggle now because while it's easy to change a line of computer code and say, reduce a transmission rate by 80 percent, it's a lot harder to do that in the real world. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.

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