New research suggests the effectiveness of flu shots fades quicker in obese people than people of normal weight.

What's more, obese people produce defective version of "killer cells" that help stop infections from worsening and spreading throughout the body, the study found.

That might help explain why obese people were more at risk of developing complications during the H1N1 pandemic two years ago.

The study is the first to find that while obese people develop the appropriate immune response in the first month after being vaccinated, their resistance appears to plunge over time.

Obese people also produce defective T-cells, the study authors report in the International Journal of Obesity.

The senior author of the study, Melinda Beck, a professor of nutrition at the University of North Carolina at Chapel Hill School of Global Public Health, says if flu vaccines offer waning protection for obese people, that puts them at risk of acquiring potentially dangerous infections.

"These results suggest that overweight and obese people would be more likely than healthy weight people to experience flu illness following exposure to the flu virus," she said in a statement.

For the study, researchers looked at 499 patients at a UNC clinic who had been vaccinated in late 2009 with the usual seasonal flu vaccine.

They found that 12 months after vaccination, about half the obese participants saw their antibody levels drop four-fold compared to one month after vaccination. But among the healthy weight people, less than a quarter saw that kind of decrease in antibody levels.

When the researchers next took blood samples and exposed them to a flu virus 12 months after vaccination, about 75 per cent of the T cells in the healthy-weight people still expressed infection-fighting proteins. But only about 25 per cent of the cells in obese patients did the same.

Co-author Patricia Sheridan said that could be an important finding because when vaccines fail to prevent infection, people must rely in part on their T cells to limit the severity of the infection.

Beck says this study adds to previous researchthat suggested obesity might impair the body's ability to fight the flu. Other studies have also found diminished responses to other vaccines in obese individuals.

During the H1N1 pandemic in 2009, doctors noticed that a high proportion of those who became severely ill had been obese. Health officials suspected that might have been due to the fact that heavy people tend to have asthma and other health conditions that make them more susceptible to complications.

Obesity in itself had never been seen as a risk factor for flu.

"These new findings seem to give us a reason why obese people were more susceptible to influenza illness during the H1N1 pandemic compared to healthy weight people," said Beck.

The authors say that further researcher should focus on what causes the large drop in antibodies to flu vaccines in the obese.

"We need to better understand this problem and to look for solutions," she said, noting that the flu kills up to half a million people a year worldwide, and rates of obesity are on the rise.