British and Chinese scientists have found a genetic variant which explains why Chinese people may be more vulnerable to the H1N1 virus, commonly known as swine flu.
It may also help explain why new strains of flu virus often emerge first in Asia.
"Understanding why some people may be worse affected than others is crucial in improving our ability to manage flu epidemics and to prevent people dying from the virus," said Tao Dong at Britain's Oxford University, who led the study.
Less than 1 percent of Caucasians are thought to have the gene alteration, which has previously been linked to severe influenza. Yet about 25 percent of Chinese people have it, and it is also common in Japanese and Korean people.
The scientists analyzed 83 patients admitted to a Beijing hospital during the 2009-2010 swine flu pandemic. Of those with serious complications such as pneumonia, respiratory or kidney failure, 69 percent had the genetic alteration. Among patients with mild illness, only 25 percent did.
"It doesn't mean you should panic if you have this gene variant," said Andrew McMichael, director of the Weatherall Institute of Molecular Medicine at Oxford University. "Most people who have it won't run into any trouble at all."
But he said people with the genetic predisposition to severe flu should be treated earlier and more aggressively than others.
McMichael estimated people with the variant were five to six times more likely to become severely ill once infected. But the gene alteration doesn't make people more likely to catch the flu, since that depends on other factors.
McMichael said the gene variant might give people the same susceptibility to get severely ill from other ailments. But it could also provide them with better immunity once they recover.
"The bug in someone who gets severely ill is not any different than the one that infects someone who has mild illness," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Genetic screening might one day be included in national flu plans.
"Further work needs to be done to justify that, but maybe in the future we would be able to say that if you're of a certain ethnicity, you are more at risk and should be prioritized for vaccination or antivirals," said Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London.
"It's possible we could one day do a genetic test before treating someone with flu to see what the best treatment would be."