(The Associated Press) - A new study claims that some future malaria vaccines that don't entirely wipe out the disease might unintentionally serve to make it more deadly.

The study, published Dec. 13 in the journal Nature, uses mathematical analysis to examine the effect of partially effective, or "imperfect," malaria vaccines.

In some scenarios, the study says, a hypothetical vaccine designed to reduce the growth rate or transmission of the malarial parasite caused it to evolve into a more virulent strain.

"The thrust of our paper is that they (parasites) can become more nasty, and certain types of vaccines can promote the nastiness of the disease," said Andrew Read, a researcher at the Institute of Cell, Animal and Population Biology at the University of Edinburgh in England.

On the other hand, vaccines specifically designed to block infection resulted in less virulent strains.

The researchers examined malaria because it provides a good example of a disease likely to require "imperfect" vaccines, Read said.

"We're now moving into an era of vaccines which deal with diseases where natural immunity isn't that good to begin with," he said. "The vaccines aren't going to be that good, and malaria is a classic example of that; the malaria vaccines probably won't be very good."

Scientists not connected to the study cautioned that it is based on a theoretical model, which uses untested assumptions that may be incorrect. They noted that no human population vaccinated for any disease has shown evidence of what Read's team posits.

"I think of this as adding one more item to the list of things that we should watch for as vaccines are used," said Dr. Marc Lipsitch, an expert in epidemiology at the Harvard School of Public Health. "It raises a theoretical possibility that many people may not have thought of before ... (but) it's not actually sounding an alarm that something is happening or is about to happen."

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