NEW YORK, United States, Tuesday November 4, 2014 – While the consensus among scientists studying the potential spread of Ebola in the United States indicates that more cases are likely by year-end it is impossible to calculate how many infections will emerge there or anywhere else.
Experts have nevertheless made informed guesses based on data models incorporating hundreds of variables, including the number of daily new infections in West Africa, air and sea traffic worldwide, and transmission possibilities.
Last week, several leading infectious disease experts ran simulations that predicted a best case scenario of as few as one or two additional infections by the end of this year, to a worst-case scenario of 130 new cases.
With a reassuring best case scenario, Northeastern University professor Alessandro Vespignani projected between a single case and a small chance of as many as eight cases by the end of this month.
The probability of international spread outside the African region is small, but not negligible, Vespignani and a team of colleagues said in an article in the journal PLOS ONE.
In the long term, they said that international dissemination will depend on what happens in West Africa in the next few months.
The team’s first analysis, published just over a month ago, proved to be accurate when it included the US among 30 countries likely to see some Ebola cases. At that time, they projected one or two infections in the US, but said there could be as many as 10.
As events unfolded, nine Ebola patients were treated in the US, with one death. Seven of them were infected in West Africa, including Liberian national Thomas Duncan, the first to arrive in the US undiagnosed and the first to die. He was cared for at a Dallas hospital where two of his nurses subsequently became infected.
Longer range projections are dependent on developments in West Africa. The best case scenario is that the surge in assistance to the affected region brings the epidemic under control, prompting cases to peter out in the US. The worst case scenario involves Ebola spreading unchecked across international borders.
As Vespignani said: “My worry is that the epidemic might spill into other countries in Africa or the Middle East, and then India or China. That could be a totally different story for everybody.”
Vespignani’s concern is shared by Dr Ashish Jha, a Harvard University professor and director of the Harvard Global Health Institute, who said he’s not worried about a handful of new cases in the US. His greatest worry is if the disease goes from West Africa to India.
“If the infection starts spreading in Delhi or Mumbai, what are we going to do?” he asked.
Meanwhile, pandemic risk expert Dominic Smith, a senior manager for life risks at Newark, California-based RMS, a leading catastrophe-modelling firm, ran a simulation last week that projected 15 to 130 cases in the US between now and year-end.
Smith’s method assumes that most cases imported to the US will be American medical professionals who worked in West Africa and returned home.
Dr David Relman, a professor of infectious disease, microbiology and immunology at Stanford University’s medical school, doubts that there will be a huge outbreak in the United States.
“However, as best we can tell right now, it is quite possible that every major city will see at least a handful of cases,” he added.
Relman is a founding member of the US Department of Health and Human Services advisory board for biosecurity and chairs the National Academy of Sciences forum on microbial threats.