[afro-nets] Vaccines May Increase (Malaria) Virulence

[Claudio Schuftan <claudio@hcmc.netnam.vn>]

Vaccines May Increase (Malaria) Virulence
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http://www.nature.com/nsu/040621/040621-3.html

Vaccines may increase virulence
Pursue additional defences, urge malaria experts.

22 June 2004
Helen Pearson

Malaria kills 2-3 million people each year. © WHO

Vaccines against malaria could cause the parasite to develop
more vicious strains. But experts warn that the finding should
not detract from the urgent hunt for a jab.

Malaria is caused by the parasite Plasmodium falciparum, which
is injected into humans by mosquitoes. Over a dozen clinical
trials are in progress for possible vaccines against the deadly
disease. Most of these jabs create conditions in which, although
the parasites can still infect people, the immune system slows
their multiplication so they do not cause disease.

To investigate whether surviving parasites change with time,
Margaret Mackinnon and Andrew Read at the University of Edin-
burgh, UK, infected a mouse with a type of Plasmodium and then
passed blood carrying the parasite on to another animal seven
days later. They repeated this process 20 times, to mimic the
ailment passing from person to person.

Parasites that moved from one vaccinated animal to another
evolved into nastier strains than those grown in non-vaccinated
animals, the researchers show in PLoS Biology1. The vaccinated
animals stayed healthy, but when the parasite they carried was
transferred into other mice, it killed more red blood cells and
made them lose more weight than the original malaria strain.

Mackinnon and Read believe that malaria vaccines could have the
same effect in people. Perhaps over decades, P. falciparum might
evolve into a more deadly form in vaccinated people, which would
pose a greater threat than ever to those unprotected by a jab.

No magic bullet

Malaria vaccines are still vital, stresses Read, because people
who are immunized will be protected from the disease. But he
urges public health officials to pursue other methods to elimi-
nate malaria, such as distributing nets and developing new
drugs, even as jabs are developed. "You shouldn't think of vac-
cines as a magic bullet," he says.

Researchers might also avoid types of vaccine that allow the
parasite to survive at low levels, he suggests. Instead, they
could focus on classes of vaccine that hobble the parasite be-
fore it infects red blood cells or which cripple it in the mos-
quito and so stop it passing from one person to another.

Many of the vaccines under trial already take the latter ap-
proach. In fact experts predict that an effective malaria vac-
cine will probably trigger the immune system into attacking the
parasite at several different stages of its life cycle.

But some experts are concerned that the new finding will under-
mine their efforts to stem a disease that kills 2 million to 3
million people each year. "It has no relevance to vaccine devel-
opment," says Adrian Hill who is working on malaria jabs at the
University of Oxford, UK.

Read disagrees: "The big question is, would the same thing hap-
pen with another virus?" If so, jabs against other diseases,
such as measles, might also have promoted the emergence of more
virulent strains. It is hard to tell whether this has happened,
because improved medical practices make it difficult to compare
death rates at a time before vaccination with those today.

References:
Mackinnon, M .J. & Read, A. F. PloS Biology, published online,
doi:10.1371/journal.pbio.0020230 (2004).