Flu viruses can swap many genes rapidly to make new resistant strains, US researchers have found.
There are many different strains of flu virus
Scientists previously believed that gene swapping progressed gradually from season to season.
The National Institutes of Health team found instead, influenza A exchanged several genes at once, causing sudden and major changes to the virus.
The findings in PLOS Biology suggest strains could vary widely each season, making it potentially harder to treat.
They also increase concerns about bird flu mutating to spread readily between humans.
Each year, experts must predict which strains will be most common and design new vaccines to fight them.
Dr David Lipman and colleagues looked at strains of influenza A that had circulated between 1999 and 2004 in New York.
These strains had given rise to the so-called Fujian strain H3N2 that caused a troublesome outbreak in the 2003-2004 flu season because the vaccine made that winter was a poor match for the virus.
Dr Lipman's team found wide variations in the 156 strains that they analysed.
Some of the strains had at least four gene swaps that had occurred in a short time period.
"The genetic diversity of influenza A virus is therefore not as restricted as previously suggested," said the researchers.
This suggests that scientists need to study circulating flu viruses more carefully because important mutations can occur suddenly and without warning, they said.
Threat of an outbreak
Scientists have been particularly worried recently about avian flu mutating and acquiring the ability to spread from human to human.
If it does, it could kill millions worldwide.
Last week, the UK government announced it would stockpile two million doses of vaccine to combat the H5N1 strain of bird flu currently circulating in Asia to protect key medical and emergency workers across Britain against a possible global pandemic.
Dr Maria Zambon, flu expert at the Health Protection Agency said: "This research confirms the genetic diversity of influenza viruses and underscores potential for reassortment."
Professor John Oxford, a virologist at Queen Mary's School of Medicine, said: "Their work shows that, overall, the virus is a lot more busy swapping genes than we ever thought it was.
"The situation could be similar in the bird flus as well."
Dr John Moore-Gillon, spokesman for the British Lung Foundation, said: "We need to find a way to attack the flu virus so it will tackle a wider range of virus.
"Currently, flu vaccination is very narrow. We have to predict what the strains are going to be, then make the vaccine.
"This work shows that the virus is wider than we thought."