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Vaccines may increase virulence

June 22, 2004 By Helen Pearson This article courtesy of Nature News.

Pursue additional defenses, urge malaria experts.

Vaccines against malaria could cause the parasite to develop more vicious strains. But experts warn that the finding should not detract from the urgent hunt for a jab.

Malaria is caused by the parasite Plasmodium falciparum, which is injected into humans by mosquitoes. Over a dozen clinical trials are in progress for possible vaccines against the deadly disease. Most of these jabs create conditions in which, although the parasites can still infect people, the immune system slows their multiplication so they do not cause disease.

To investigate whether surviving parasites change with time, Margaret Mackinnon and Andrew Read at the University of Edinburgh, UK, infected a mouse with a type of Plasmodium and then passed blood carrying the parasite on to another animal seven days later. They repeated this process 20 times, to mimic the ailment passing from person to person.

Parasites that moved from one vaccinated animal to another evolved into nastier strains than those grown in non-vaccinated animals, the researchers show in PLoS Biology1. The vaccinated animals stayed healthy, but when the parasite they carried was transferred into other mice, it killed more red blood cells and made them lose more weight than the original malaria strain.

Mackinnon and Read believe that malaria vaccines could have the same effect in people. Perhaps over decades, P. falciparum might evolve into a more deadly form in vaccinated people, which would pose a greater threat than ever to those unprotected by a jab.

No magic bullet

Malaria vaccines are still vital, stresses Read, because people who are immunized will be protected from the disease. But he urges public health officials to pursue other methods to eliminate malaria, such as distributing nets and developing new drugs, even as jabs are developed. "You shouldn't think of vaccines as a magic bullet," he says.

Researchers might also avoid types of vaccine that allow the parasite to survive at low levels, he suggests. Instead, they could focus on classes of vaccine that hobble the parasite before it infects red blood cells or which cripple it in the mosquito and so stop it passing from one person to another.

The big question is, would the same thing happen with another virus?
Andrew Read
University of Edinburgh
Many of the vaccines under trial already take the latter approach. In fact experts predict that an effective malaria vaccine will probably trigger the immune system into attacking the parasite at several different stages of its life cycle.

But some experts are concerned that the new finding will undermine their efforts to stem a disease that kills 2 million to 3 million people each year. "It has no relevance to vaccine development," says Adrian Hill who is working on malaria jabs at the University of Oxford, UK.

Read disagrees: "The big question is, would the same thing happen with another virus?" If so, jabs against other diseases, such as measles, might also have promoted the emergence of more virulent strains. It is hard to tell whether this has happened, because improved medical practices make it difficult to compare death rates at a time before vaccination with those today.


  1. Mackinnon M .J., & Read A. F. PloS Biology, published online, doi:10.1371/journal.pbio.0020230 (2004).


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