Alessandro Vespignani hopes his latest book will prove to be false. In July, the physicist at Northeastern University in Boston began modeling how the deadly Ebola virus can spread in West Africa. Extrapolating current trends, the number of sick and dying quickly rises of more than 3000 cases tolls current and 1500 deaths at about 10,000 cases before September 24, and hundreds of thousands in the following months. "The numbers are really scary," he said, although he stressed that the model assumes control efforts are not strengthened. "We all hope to see what is not happening," Vespigani wrote in an e-mail .
Vespignani is not alone in trying to predict how the unprecedented epidemic will progress. last week, the World health Organization (WHO) estimates that the number of cases could ultimately exceed 20,000. and scientists around the world are scrambling to create computer models that accurately describe the spread of the deadly virus. all of them seem not quite as dark as Vespignani of. But all modelers agree that current efforts to fight against the epidemic are not enough to stop the deadly pathogen in its tracks.
the computer models "are incredibly helpful" in the fight against an epidemic, says researcher of infectious diseases Jeremy Farrar, who leads the Wellcome Trust research organization in London. They can help organizations such as WHO predict medical supplies and personnel they need and can indicate what interventions will be better contain the epidemic. Christian Althaus mathematical epidemiologist at the University of Bern, which is also built models of Ebola, says WHO and Samaritan's Purse, a relief organization fight against Ebola, contacted him to learn about its projections.
But modelers are hampered by the lack of data on the current epidemic and the lack of knowledge about how the Ebola virus spreads. The funerals of Ebola victims are known to spread the virus, for example, but how many people are infected in this way is not known. "Before that, we never had much of Ebola, so that epidemiology has not been well developed," says Ira Longini, a biostatistician at the University of Florida in Gainesville. "We are caught with our pants down."
To a mathematician, the fight against any epidemic is at its core a struggle to reduce a number: R e , the effective reproductive rate the pathogen, the number of people a person infected in turn infects on average. E R 1 above, and the disease spreads. Below 1, an epidemic will stall
The Outbreak models typically assume that there are four groups of people :. Those who are susceptible, those who were infected but not yet infectious, those who are sick and can transmit the virus, and those who have recovered. A model, in essence, describes the rate at which people move from one group to another. Thereof, R e can be calculated.
If the disease continues to spread as it has, most modellers Science spoke to say WHO estimate will prove to be conservative. "If the epidemic in Liberia should continue this until 1 December, the cumulative number of cases exceeds 100,000," Althaus predicted. These long-term forecasts are mistakes, he admits. But other modelers are not much more encouraging. Caitlin Rivers Institute and State University of Blacksburg Virginia Polytechnic to expect about 1,000 new cases in Liberia in the next 2 weeks and a similar number in Sierra Leone.
Vespignani analyzed the probability that the Ebola virus will spread to other countries. Using data on millions of air travelers and commuters and mobility patterns based on census data and mobile devices, he built a model of the world in which he can introduce the Ebola virus, then run hundreds of thousands of simulations. In general, the risk of spread beyond West Africa is small, Vespignani said, but the risk increases with the magnitude of the epidemic. Ghana, the United Kingdom and the United States are among the most likely to have a case presented country, depending on model. (Senegal, which reported its first case of Ebola last week, was in ten countries, too.)
The models are only as good as the data entered for them; up to three quarters of Ebola cases may go unreported. Modelers are also assuming that the key parameters, such as the incubation time of the virus are the same as previously occurred. "We could miss the boat and we have no signal to indicate that," said Martin Meltzer of the US Centers for Disease Control and Prevention in Atlanta.
The biggest uncertainty is how many doctors, nurses and others can slow the virus. There are several ways to push down R e , Farrar said hand washing, wearing masks, or quarantine of people, for example. "But given the complexity of the epidemic and limited resources, we need to know what are the two or three things that go help reduce infections," says Farrar, and that is where models can help. For example, would track all contacts all cases be more effective than monitoring the much smaller number that had some type of contact with a case, such as sharing a room?
evaluates Rivers current interventions such as increased use of protective equipment or campaigns to isolate infected people. in the most optimistic scenario, all the contacts of infected people is traced, and transmission in hospitals is reduced by 75 %. Even that, while significantly reducing the number of deaths from Ebola did not push R e below a.
the challenge varies by country, said Althaus . "in Guinea and Sierra Leone, R e is close to 1 and the epidemic could be stopped if the interventions improve a bit." in Liberia, R e was near 1.5 all the time. "This means that the work is just beginning here." But Meltzer says there is no reason to believe that the situation is better in Sierra Leone. "We see no change in the rate of accumulation of cases "he said.
as the models will better differentiate what happens in places, Rivers said," you might be able to stop lines around certain communities. "But these measures are highly controversial. When Liberia last week barricaded off West Point, a sprawling slum with probably more than 100,000 inhabitants, it attracted a largely negative response. "quarantines and curfews tend to instill fear and mistrust to the entire response, including health facilities, "said a representative of Médecins Sans Frontières Science . Paul Seabright, a researcher at the Toulouse School of economics in France, which studied these measures, said they are an incentive for people to keep a secret if they have been in contact with a patient. Liberia tough actions are "the last thing this epidemic needs," he said.
The people in West Africa will change behavior, Meltzer said. "We will not stop this epidemic only by building hospitals. There should be a change in how the community deals with the disease. "Modeling is easy enough, Vespignani said." I can reduce the transmission at the funeral of 40% easily in a model. It's a line of code. But in the area that is really hard "
* Ebola files :. Given the current Ebola epidemic, unprecedented in terms of the number of people killed and the rapid geographic spread, Science and Science Translational Medicine made a collection of research articles and news on the viral disease available for researchers and the general public.
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