Study provides new understanding of forces behind seasonal flu virus evolution

Do influenza viruses persist in low levels year-round in the northern and southern hemispheres, or does a new crop of the virus emerge afresh in tropical zones such as Southeast Asia before spreading into temperate regions around the globe" Researchers have provided an answer to this long-standing question: new strains arise each year.

The new findings should help public health officials more quickly and accurately determine which strains to include in the annual flu vaccine.

The study, supported in part by the National Institutes of Health (NIH), appears online in advance of print in the journal Nature.

The researchers analyzed full gene sequences of seasonal influenza virus samples collected from the world's temperate regions north and south of the equator. Their data comprised full genetic sequences of 1,302 isolates of influenza A virus collected over 12 years from New Zealand and New York state.

By quantifying the degree of genetic diversity among the strains' subtypes, gene segments and geographic locations, the researchers were able to detect patterns indicating that virus strains do not persist from one flu season to the next in the temperate regions. Therefore, the researchers deduced, new flu strains emerge annually from the tropics.

◊ A public release from NIH/National Institute of Allergy and Infectious Diseases in April 2008, viewed from Biology-Online.org.