Researchers model evolution of virus.(severe acute respiratory syndrome cause sought)(influenza virus evolution analyzed)

2003 APR 16 - (NewsRx.com & NewsRx.net) -- As health agencies around the world race to pinpoint the cause of severe acute respiratory syndrome (SARS), researchers are reporting success in developing a new theoretical model that shows how the pressure exerted by the immune response of an infected population can drive evolution of influenza virus.

The model does not aim to predict the emergence of new strains of influenza, but it does suggest that a short-lived general immunity to the virus might affect the virus's evolution. If immunologists can understand the basis of such a response by influenza virus, then vaccine designers might use that understanding to develop a vaccine that offers more general immunity to the virus, said the scientists.

The researchers - led by Howard Hughes Medical Institute international research scholar Neil M. Ferguson at Imperial College London - published an article outlining their model in Nature. Coauthors are Alison Galvani from the University of California at Berkeley, and Robin Bush from the University of California, Irvine.

"The principal question we were trying to address with this model is what biological factors determine the particular patterns we see in influenza evolution," said Ferguson. "We wanted to understand the role of immunity in determining the competition between different flu strains."

Strains of flu virus differ from one another largely in the genes that code for surface molecules called glycoproteins, which are the primary targets of the body's immune system in defending against flu viruses, said Ferguson. Evolutionary changes in immune response against such "antigen" molecules are the reason that new vaccines must be developed against emerging strains of virus.

A central mystery, said Ferguson, was why only a few new flu strains emerge over time, replacing other strains that go extinct. Limitations on genetic variance distinguish influenza from other RNA viruses such as HIV and dengue fever, which exist in a wide range of variants, he said.

"Given basic evolutionary theory, one might expect naively that new influenza strains wouldn't necessarily drive the others extinct, and the virus population would get more and more diverse," he said. "Understanding what stops that happening was the key question posed in this study."