Vaccines breed viciousness
Vaccinations may increase death
toll.
13 December 2001
HELEN PEARSON
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Vaccines can
drive the evolution of virulent disease.
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© Photodisc
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Inadequate vaccines can encourage the emergence of nastier bugs, placing
the unprotected at risk, a new mathematical model shows. The effect could
undermine future vaccination programmes.
Many vaccines save people from dying of a disease, but do not stop them
carrying and transmitting it. Over a few decades this may cause more virulent
strains to evolve, predict Andrew Read and his colleagues of the University
of Edinburgh, UK1.
In some situations, such as in areas endemic for malaria, deadlier
disease strains could kill more people than vaccination saves. "Most
of the time the benefits [of vaccination] will be eroded," says Read.
Vaccines for HIV, and hepatitis B and C "give most cause for
concern", says immunologist Charles Bangham, of Imperial College in
London. These viruses are difficult for the body's immune system to
eradicate, leaving them time to reproduce and evolve. Tearaway strains of
flu also emerge regularly and evade existing vaccines.
Infections that linger in the body are more likely to meet a second bug,
explains evolutionary biologist Dieter Ebert from the University of
Fribourg in Switzerland. The competition drives pathogens to evolve faster,
nastier killing tactics to get the most from their host.
Don't encourage them
Vaccines that encourage evolution include those that slow a
disease-causing organism's growth or target its harmful toxin. These types
are being pursued to fight diseases such as anthrax and malaria. The
possibility that these might save individuals but harm populations
"has not been considered before", says Ebert, and should be a
factor in public-health policy.
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New vaccines should aim to
prevent pathogens getting a toehold
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Charles
Bangham, Imperial College London
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Most existing vaccines, such as those for smallpox, polio and measles,
are very effective as they use a different strategy. They stimulate a
natural immune reaction which either kills off subsequent infections or
blocks pathogen reproduction and transmission altogether. Read does not
advocate halting such programmes. New vaccines should similarly aim to
prevent pathogens getting a toehold, says Bangham; many in the pipeline do
not.
Several different vaccines are being developed to fight malaria: results
of clinical trials for one that interrupts the life cycle of microorganism Plasmodium
falciparum were announced last week2. 'Multivalent
vaccines' that target several different parts of a pathogen or life cycle
at once are the better choice, Read suggests.
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