NIH STUDY HAS IMPLICATIONS FOR FLU VACCINE FORMULATION

U.S. researchers have reported results of a study that substantially alters the existing understanding of how the influenza virus evolves and that could have important implications for predicting which strains should be used for flu vaccine. The study, published Oct. 26 in the online journal Biology Direct, was conducted by researchers from the NIH.

In an effort to better understand how seasonal influenza evolves into new strains, the researchers analyzed the genomic sequences of a large and representative collection of the two most common flu strains (called H3N2 and H1N1) from the 1995 through 2005 flu seasons in New York state and New Zealand. The sequence data was obtained from the Influenza Genome Sequencing Project, which recently generated over 1,000 fully sequenced influenza genomes from clinical isolates.

The analysis revealed a picture of flu evolution that was surprisingly different from the prevailing conception of how the virus changes, researchers said. Evolution of influenza A virus is commonly viewed as a typical Darwinian process. However, the study found that the periods of intense Darwinian selection accounted for only a relatively small portion of H3N2 flu evolution during the 10-year period examined.

Researchers concluded that "the common view of the evolution of influenza virus as a rapid, positive selection-driven process is, at best, incomplete." Because the periods of stasis allow the proliferation of many small groups of related viruses, any of which could become the next dominant virus strain, the authors suggest that sequencing much larger numbers of representative isolates could be helpful in augmenting current surveillance methods.