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 So-called "asymptomatic" Ebola cases - in which someone is exposed
to the virus, develops antibodies, but doesn't get sick or suffer
symptoms - are hotly disputed among scientists, with some saying
their existence is little more than a pipe dream.
Yet if, as some studies suggest, such cases do occur in epidemics of
the deadly disease, they may be a key factor in ending outbreaks
more swiftly by giving secret protection to those lucky enough to be
able to bat the infection away.
"We wonder whether 'herd immunity' is secretly coming up - when you
get a critical mass of people who are protected, because if they are
asymptomatic they are then immune," Philippe Maughan, senior
operations administrator for the humanitarian branch of the European
Commission, told Reuters. "The virus may be bumping into people it
can't infect any more."
Latest World Health Organization data show new cases of infection in
West Africa's unprecedented Ebola epidemic dropping dramatically in
Guinea, Sierra Leone and particularly in Liberia.
 Most experts are sure the main driver is better control measures
reducing direct contact with contagious patients and corpses, but
there may also be other factors at work.
So-called herd immunity is a feature of many infectious diseases and
can, in some cases, dampen an outbreak if enough people get
asymptomatic, or "sub-clinical" cases and acquire protective
antibodies. After a while, the virus - be it flu, measles, polio -
can't find non-immune people to be its hosts.
But some specialists with wide experience of disease outbreaks are
highly skeptical about whether this phenomenon happens in Ebola, or
whether it could affect an epidemic.
"There is some suggestion there may be cases that are less severe...
and there may even be some that are asymptomatic," said David
Heymann, an infectious disease expert and head of global health
security at Chatham House.
"But herd immunity is just the wrong term. There could be household
immunity developing, but even that is only hypothesis."
Others are more hopeful and are urging researchers in West Africa to
seek out and test possible asymptomatic cases with a view to using
the secrets of their silent immunity.
Steve Bellan of the University of Texas in the United States argues
that if scientists can reliably identify asymptomatic people, they
could help with disease-control tasks like caring for patients and
conducting burials, reducing the number of non-immune people exposed
in these risky jobs.
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Bellan points to two studies in particular. One, conducted after an
Ebola outbreak in Gabon in 1997, found that 71 percent of
"seropositive" people - those with traces of the Ebola virus in
their blood - did not have the disease. The other, published in
April 2002, found 46 percent of asymptomatic close contacts of
patients with Ebola were seropositive.
With the largest Ebola epidemic on record raging through three of
Africa's most under-resourced countries, scientists and medics have,
understandably, focused all efforts on the sick and dying and not on
testing people with no symptoms.
If they do, however, and if they were to find what Bellan and some
others suspect, it could prompt a reappraisal of what jolted a
relatively sudden downturn in new cases.
Some researchers say they have identified a few cases in the current
outbreak with mild symptoms and low concentrations of Ebola virus in
the blood. One was a Guinean student who traveled to Senegal and is
not known to have infected anyone else, despite having contact with
dozens of people.
Ian MacKay, a virologist at Australia's University of Queensland,
agrees that possible sub-clinically-acquired immunity is one of many
unexplored mysteries of the Ebola virus.
"One thing that this particular outbreak shows is that we really
don't know an awful lot about these kinds of infectious diseases,"
he said. "We tend to think we can answer all the questions, but this
is one of those things we may end up being taught by the virus
itself."
(Editing by Peter Graff)
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