Number of Ebola cases in U.S. by end of year hard to predict

STANFORD, Calif. – Top medical experts studying the spread of Ebola say the public 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.

But how many cases?

No one knows for sure how many infections will emerge in the U.S. 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.

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 U.S. 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 Centers for Disease Control and Prevention have focused on worst-case scenarios for West Africa, concluding that cases in the U.S. 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 but contagious only through contact with bodily fluids, requiring no more than 21 days for symptoms to emerge. Human behavior 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-modeling firm, ran a U.S. simulation last week that projected 15 to 130 cases between now and the end of December. That’s less than one case per 2 million people.

Smith’s method assumes that most cases imported to the U.S. will be American medical professionals who worked in West Africa and returned home.

Smith said the high end may be an overestimate as it does not include the automatic quarantining measures that some areas in the U.S. 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 – U.S. 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.

In an article in the journal PLOS ONE, Vespignani and a team of colleagues said the probability of international spread outside the African region is small but not negligible. Longer term, they say international dissemination will depend on what happens in West Africa in the next few months.

Their first analysis, published Sept. 2, proved to be accurate when it included the U.S. among 30 countries likely to see some Ebola cases. They projected one or two infections in the U.S., but there could be as many as 10.

So far, nine Ebola patients have been treated in the U.S. and one has died. The foreseeable future extends only for the next few months. After that, projections depend entirely on what happens in West Africa. One scenario is that the surge in assistance to the region brings the epidemic under control and cases peter out in the U.S. A second scenario involves Ebola spreading unchecked across international borders.

“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 said.

The Centers for Disease Control and Prevention prefers not to focus on a particular number. But spokeswoman Barbara Reynolds said Ebola will not be a widespread threat as some outside the agency have warned.

“We’re talking about clusters in some places but not outbreaks,” she said.

The CDC is using modeling tools to work on projections in West Africa, but “there isn’t enough data available in the U.S. to make it worthwhile to go through the exercise.”