Nearly a quarter of ethnic Chinese have a tiny genetic variant that boosts sixfold their risk of falling gravely ill when infected with flu, a study published on Tuesday said.

Researchers in China and Britain looked at Chinese hospital data from the 2009-2010 pandemic of H1N1 influenza.

Patients who had the minute variant were six times likelier to be treated for severe infection compared with patients who had a different genetic type, they found.

The variation focuses on just a single change in the code of a key gene in the immune system.

The IFITM3 gene -- for "interferon-induced transmembrane protein-3" -- determines how cells fight the influenza virus.

The single code change is called rs12252, and it comes in three variants, also called genotypes: CC, CT and TT.

Around 24 percent of Han Chinese have the CC genotype, but their numbers were disproportionately high among those who were badly affected by flu, the investigators learned.

Among those who were treated for serious symptoms and whose genetic code had been sequenced, 59 percent had the CC genotype.

"The CC genotype was estimated to confer a sixfold greater risk for severe infection than the CT and TT genotypes," said the paper, appearing in the journal Nature Communications.

Risk of dying from flu is also "probably" higher for the CC genotype although the probe did not specifically address this question, said Andrew McMichael, who partnered Tao Dong at the University of Oxford in the British side of the investigation.

Between 25 and 50 percent of cases of severe infection required assisted breathing, he noted in an email exchange with AFP.

The vulnerability of the CC genotype also probably applies to all strains of the flu virus, and not just H1N1, said McMichael.

Only 0.03 percent of northern Europeans have the CC variant, and they too are disproportionately more at risk than other genotypes. This sub-group accounted for 5.3 percent of hospital admissions for all strains of flu, previous research has found.

The study calls for urgent work to see whether genetic screening could be useful for Chinese, or other Asians, who could be most at risk.

"It might help at the time of admission to hospital to help predict the course of infection and therefore treatment, but this needs to be assessed very carefully by our Chinese colleagues in future studies," said McMichael.