Strains of the influenza virus are constantly swapping genes among themselves and giving rise to new, dangerous strains at a rate faster than previously believed, U.S. researchers reported on Monday.

They found that slightly mutated influenza A strains in New York that circulated between 1999 and 2004 gave rise to the so-called Fujian strain that caused a troublesome outbreak in the 2003-2004 flu season.

Such events probably are what lead to the occasional pandemics of flu that can kill millions of people, David Lipman and colleagues at the National Institutes of Health found.

They hope their findings, published in the journal Public Library of Science Biology, will help scientists better predict which viral strains will attack during upcoming flu seasons and design better vaccines.

Influenza viruses are notorious for trading genes back and forth and mutating. Scientists previously believed that the gene swapping occurred gradually but the new study shows that several genes can be exchanged at once, causing sudden changes in important characteristics of the virus.

This is why a new flu epidemic sweeps the world every year, killing between 250,000 and 500,000 globally and 36,000 people in the United States alone every year.

Each year, experts must predict which strains will be most common and design a new vaccine to fight them. Some years, such as in 2003-2004, the vaccine does not include the most common strain.

Lipman and colleagues sequenced the genomes of 156 influenza A viruses, named H3N2, that were collected by New York State public health officials between 1999 and 2004.

“We found that there are co-circulating minor variants that are not infecting many people,” Lipman said in a statement. “One of these can cause the next major epidemic.”

They found “at least four reassortment events occurred among human viruses during the period 1999-2004” — meaning there was an exchange of genes four different times.

By Maggie Fox

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