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2009-07-30 08:36:33
By MARILYNN MARCHIONE, AP Medical Writer Marilynn Marchione, Ap Medical Writer
Thu Jul 30, 12:13 am ET
In a daring experiment in Europe, scientists used mosquitoes as flying needles
to deliver a "vaccine" of live malaria parasites through their bites. The
results were astounding: Everyone in the vaccine group acquired immunity to
malaria; everyone in a non-vaccinated comparison group did not, and developed
malaria when exposed to the parasites later.
The study was only a small proof-of-principle test, and its approach is not
practical on a large scale. However, it shows that scientists may finally be on
the right track to developing an effective vaccine against one of mankind's top
killers. A vaccine that uses modified live parasites just entered human
testing.
"Malaria vaccines are moving from the laboratory into the real world," Dr.
Carlos Campbell wrote in an editorial accompanying the study in Thursday's New
England Journal of Medicine. He works for PATH, the Program for Appropriate
Technology in Health, a Seattle-based global health foundation.
The new study "reminds us that the whole malaria parasite is the most potent
immunizing" agent, even though it is harder to develop a vaccine this way and
other leading candidates take a different approach, he wrote.
Malaria kills nearly a million people each year, mostly children under 5 and
especially in Africa. Infected mosquitoes inject immature malaria parasites
into the skin when they bite; these travel to the liver where they mature and
multiply. From there, they enter the bloodstream and attack red blood cells
the phase that makes people sick.
People can develop immunity to malaria if exposed to it many times. The drug
chloroquine can kill parasites in the final bloodstream phase, when they are
most dangerous.
Scientists tried to take advantage of these two factors, by using chloroquine
to protect people while gradually exposing them to malaria parasites and
letting immunity develop.
They assigned 10 volunteers to a "vaccine" group and five others to a
comparison group. All were given chloroquine for three months, and exposed once
a month to about a dozen mosquitoes malaria-infected ones in the vaccine
group and non-infected mosquitoes in the comparison group.
That was to allow the "vaccine" effect to develop. Next came a test to see if
it was working.
All 15 stopped taking chloroquine. Two months later, all were bitten by
malaria-infected mosquitoes. None of the 10 in the vaccine group developed
parasites in their bloodstreams; all five in the comparison group did.
The study was done in a lab at Radboud University in Nijmegen, the Netherlands,
and was funded by two foundations and a French government grant.
"This is not a vaccine" as in a commercial product, but a way to show how whole
parasites can be used like a vaccine to protect against disease, said one of
the Dutch researchers, Dr. Robert Sauerwein.
"It's more of an in-depth study of the immune factors that might be able to
generate a very protective type of response," said Dr. John Treanor, a vaccine
specialist at the University of Rochester Medical Center in Rochester, N.Y.,
who had no role in the study.
The concept already is in commercial development. A company in Rockville, Md.
Sanaria Inc. is testing a vaccine using whole parasites that have been
irradiated to weaken them, hopefully keeping them in an immature stage in the
liver to generate immunity but not cause illness.
Two other reports in the New England Journal show that resistance is growing to
artemisinin, the main drug used against malaria in the many areas where
chloroquine is no longer effective. Studies in Thailand and Cambodia found the
malaria parasite is less susceptible to artemisinin, underscoring the urgent
need to develop a vaccine.