Scientists try to predict number of US Ebola cases
STANFORD, California (AP): Top medical experts studying the spread of Ebola say Americans should expect more cases to emerge in the United States by year's end as infected people arrive here from West Africa, including American doctors and nurses returning from the hot zone and people fleeing from the deadly disease.
No one knows for sure how many infections will emerge in the US or anywhere else, but scientists have made educated guesses based on data models that weigh hundreds of variables, including daily new infections in West Africa, airline traffic worldwide and transmission possibilities.
This week, several top infectious disease experts ran simulations for The Associated Press that predicted as few as one or two additional infections by the end of 2014 to a worst-case scenario of 130.
"I don't think there's going to be a huge outbreak here, no," said Dr David Relman, a professor of infectious disease, microbiology and immunology at Stanford University's medical school. "However, as best we can tell right now, it is quite possible that every major city will see at least a handful of cases."
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.
Until now, projections published in top medical journals by the World Health Organization and the US Centers for Disease Control have focused on worst-case scenarios for West Africa, concluding that cases in the US will be episodic, but minimal. But they have declined to specify actual numbers.
The projections are complicated, but Ebola has been a fairly predictable virus — extremely infectious, contagious only through contact with body fluids, requiring no more than 21 days for symptoms to emerge. Human behaviour is far less predictable — people get on airplanes, shake hands, misdiagnose, even lie.
Pandemic risk expert Dominic Smith, a senior manager for life risks at Newark, California-based RMS, a leading catastrophe-modelling firm, ran a US simulation this week that projected 15 to 130 cases between now and the end of December. That's less than one case per two million people.
Smith's method assumes that most cases imported to the US will be American medical professionals who worked in West Africa and returned home.
Smith said the high end may be a bit of an overestimation, as it does not include the automatic quarantining measures that some areas in the US are implementing.
Those quarantines "could both reduce the number of contacts for imported cases, as well as increase the travel burden on — and perhaps reduce the number of — US volunteers planning to support the effort in West Africa", he said.
In a second simulation, Northeastern University professor Alessandro Vespignani projected between one case — the most likely scenario — and a slim chance of as many as eight cases though the end of November.
"I'm always trying to tell people to keep calm and keep thinking rationally," said Vespignani, who projects the spread of infectious diseases at the university's Laboratory for the Modeling of Biological and Socio-Technical Systems.