Vaccines may increase virulence

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http://www.nature.com/nsu/040621/040621-3.html

 

Vaccines may increase virulencePursue additional defences, urge malaria experts.

22 June 2004

HELEN PEARSON

 

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.

 

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.

References

 

Mackinnon, M .J. & Read, A. F. . PloS Biology, published online,

doi:10.1371/journal.pbio.0020230 (2004).|Article|

 

 

© Nature News Service / Macmillan Magazines Ltd 2004