When Pennsylvania State University biologist Andrew Read injected mice with a component of several promising malaria vaccines, he got a disquieting result: The malaria parasites spread through the immunized mice and evolved to become more virulent.

Unvaccinated mice infected with these super-parasites got much sicker than those infected with ordinary malaria.

The findings, Read said, should not discourage research on malaria vaccines - the disease kills hundreds of thousands of African children every year, and the parasites tend to develop resistance to drugs. Between 15 and 20 vaccines are currently in clinical trials around the world, mostly in Africa. Read, who is trained as an evolutionary biologist, said he hopes his result will prompt vaccine researchers to consider how vaccines may affect the evolution of the parasites.

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