- Inadequate vaccines can the encourage 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,"
- 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.
- New vaccines should aim to prevent pathogens getting
a toehold Charles Bangham, Imperial College London
- 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
- 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. ___
- References Gandon, S., Mackinnon, M. J., Nee, S. &
Read, A. F. Imperfect vaccines and the evolution of pathogen virulence.
Nature, 414, 751 - 756, (2001).
- Bojang, K. A. et al. Efficacy of RTS,S/AS02 malaria vaccine
against Plasmodium falciparum infection in semi-immune adult men in The
Gambia: a randomised trial. Lancet, 358, 1927 - 934, (2001).