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STANFORD, Calif. (AP) -- 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.
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 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 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, contagious only through contact with body
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 this 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 a bit of 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, eight Ebola patients have
been treated in the U.S. and one has died. Six became infected in West
Africa: three doctors, a nurse, an NBC News cameraman and Thomas Eric
Duncan, the first to arrive undiagnosed and the first to die. He was
cared for at a Dallas hospital, where two of his nurses were also
infected.
Duncan, who was initially misdiagnosed and sent home from the emergency room, is Vespignani's worst-case scenario for the U.S.
A similar situation, if left unchecked, could lead to a local cluster that could infect, on the outside, as many as 20 he said.
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," Vespitnani said.
Dr.
Ashish Jha, a Harvard University professor and director of the Harvard
Global Health Institute, said he's not worried about a handful of new
cases in the U.S. 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?"
Dr.
Peter Hotez, founding dean of the National School of Tropical Medicine
at Baylor College of Medicine and director of the Texas Children's
Hospital Center for Vaccine Development pegs the range of cases in the
U.S. between five and 100.
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."
University of Texas
integrative biology professor Lauren Ancel Meyers said there are
inherent inconsistencies in forecasting "because the course of action
we're taking today will impact what happens in the future."
Her laboratory is running projections of Ebola's spread in West Africa.
The
U.S. simulations run for the AP had fairly consistent results with each
other, she said. And they are "consistent with what we know about the
disease http://hosted.ap.org/dynamic/stories/U/US_EBOLA_HOW_BAD_CAN_IT_GET?SITE=AP&SECTION=HOME&TEMPLATE=DEFAULT&CTIME=2014-11-01-10-25-24
