The influenza vaccine may not be as effective for those who are obese as it is for others, University of North Carolina at Chapel Hill researchers suggest. The findings, published online in the International Journal of Obesity, provide evidence explaining a phenomenon first noticed during the 2009 H1N1 flu outbreak -- obesity is associated with an impaired immune response to the influenza vaccination in humans. \"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,\" senior author Melinda Beck, a professor and associate chair of nutrition at the UNC Gillings School of Global Public Health, said in a statement. \"Previous studies have indicated the possibility that obesity might impair the human body\'s ability to fight flu viruses. 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.\" Influenza vaccine antibody levels decline significantly in obese people compared to healthy weight individuals and a type of white blood cell are defective in heavier people. Beck and colleagues studied people at a clinic who had been vaccinated in late 2009 with the common flu vaccine for that fall and winter season. Although obese, overweight and healthy weight individuals all developed antibodies to flu viruses within the first month after vaccination, the antibody levels in the blood declined more rapidly in obese and overweight individuals, the study said.