[Determining the overall level of immunity in
West Africa would require testing thousands of blood samples, an impossible
task in the current chaos, especially when any slip of a needle or a broken
vial could fatally infect a health worker.]
By Donald G. Mcnell Jr.
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The Ebola virus. Credit Center for Disease Control, via
Associated Press
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Although few medical experts realize it, part
of the population in West Africa is immune to the Ebola virus, according to
virologists who specialize in the disease.
Assuming they are correct, and if those people
can be identified, they could be a great help in fighting the outbreak. Immune
persons could safely tend the sick and bury the dead just as smallpox survivors
did in the centuries before smallpox vaccine.
Also, antibodies could be harvested from their
blood to treat new Ebola victims.
But many factors remain unclear, including
which Africans have antibodies and how much antibody is needed to be
protective. The biggest mystery is how the immunity arose, and there is a mix
of explanations, like silent infections and fruit contaminated with bat saliva.
“It’s fair to say that some people are
immune,” said Robert F. Garry Jr., a Tulane University expert in hemorrhagic
fevers who works in Sierra Leone. “But we don’t know if it’s 1 percent or 2
percent or 20 percent.”
Right now, there are about 1,800 survivors of
the current West African outbreak, all of whom are now immune, of course. But
there may be many thousands more.
Small studies of household contacts of Ebola
victims show that some people are infected without ever falling ill — perhaps
because of some unknown genetic advantage.
But many Africans who have never seen a victim
also have antibodies.
It is possible that some get low doses of
virus by eating infected monkeys or bats that are undercooked.
“If someone got just two or three or four
virus particles, if it enters through the mucus membranes in the mouth, yes,
it’s plausible,” said Thomas W. Geisbert, a hemorrhagic fever expert at the
University of Texas Medical Branch in Galveston. “It would take a while for the
virus to get going, and it’s a race with the clock. The immune system gets a
chance to fight it off.”
Antibodies, Y-shaped proteins that attach to a
virus and block it from invading cells, are the immune system’s first line of
defense; the second line is white blood cells primed to recognize and digest
the virus.
One of France’s leading Ebola experts says he
believes that many rural villagers are “vaccinated” by eating fruit gnawed on
by bats and contaminated with their saliva.
“We imagine that this is the main route,” said
Dr. Eric M. Leroy, a veterinarian and virologist at the International Center
for Medical Research in Franceville, Gabon. “But it is a hypothesis. We do not
have the evidence.”
Determining the overall level of immunity in
West Africa would require testing thousands of blood samples, an impossible
task in the current chaos, especially when any slip of a needle or a broken
vial could fatally infect a health worker.
But in 2010, Dr. Leroy led such a study in
Gabon, a Central African country that had four Ebola outbreaks from 1994 to
2002.
His teams took 4,349 blood samples in 220
randomly selected villages. They found that 15 percent of Gabon’s population
had antibodies. But it varied widely: near the coast, only 3 percent did; in
some jungle villages near the Congo border, up to 34 percent did.
Also, their antibody levels varied widely, and
what level is protective is roughly known for lab monkeys, but not for humans.
“I don’t think we have a good idea of what
constitutes a person who’s going to survive versus a person who’s going to
succumb,” said Randal J. Schoepp, head of diagnostics at the United States Army
Medical Research Institute of Infectious Diseases in Fort Detrick, Md., who
led a
study of blood from patients in a Sierra Leone hospital who
were originally thought to have Lassa fever but did not. Nearly 9 percent had
Ebola antibodies — and the samples dated from as far back as 2006, proving that
the virus circulated long before this year’s outbreak.
Also, there is anecdotal evidence that some
West Africans are resistant. Victims have relatives who never get sick. At the
funeral of a traditional healer where 14 women became infected, at least 26
other mourners did not, Dr. Garry said, even though most probably touched the
body.
There is firm evidence for silent infections.
In 2000, Dr. Leroy’s team studied 24
Gabonese who had tended victims without ever falling ill. Eleven had not just
antibodies but remnants of virus and markers of inflammation in their blood —
meaning they had clearly been infected but had defeated the virus on their own.
A similar 1999 study by
American scientists in the Democratic Republic of Congo found similar results
in five of 152 household contacts.
Those who are immune can donate
blood containing antibodies to be given to acutely ill
patients, as was done for Dr. Kent Brantly, one of the first two
Americans to get Ebola. He survived, although his Emory University doctors
later said it was unclear whether the transfusion or an experimental drug,ZMapp, containing cloned antibodies,
helped him at all.
Having those who are immune be caregivers and
body carriers makes sense, said Tom Skinner, a spokesman for the Centers for
Disease Control and Prevention.
“But we can’t count on their immunity,” he
added. “They would still need full personal protective gear.”
Relying on such measures may be inevitable,
Dr. Garry said, adding: “There’s no more ZMapp out there. It’s time for
creative solutions.”
