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By Helen Pearson
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 programs.
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, UK(1).
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.
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.
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 programs.
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 week(2). 'Multivalent vaccines' that target
several different parts of a pathogen or life cycle at once
are the better choice, Read suggests.
Nature
December 13, 2001
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