Research by the University of Alberta, published in the American Journal of Respiratory and Critical Care Medicine, suggests the overall benefit of flu vaccines "appears to have been exaggerated," the American Thoracic Society said Friday in a release.

The study involved 700 matched elderly subjects, half of whom had taken the vaccine and half of whom had not.

"Previous studies were likely measuring a benefit not directly attributable to the vaccine itself, but something specific to the individuals who were vaccinated -- healthy-user benefit or frailty bias," Dean T. Eurich of the School of Public Health at the University of Alberta said. "Over the last two decades in the United Sates, even while vaccination rates among the elderly have increased from 15 to 65 percent, there has been no commensurate decrease in hospital admissions or all-cause mortality."

Dr. Sumit Majumdar, principal investigator in the study, said people with chronic respiratory diseases, immuno-compromised patients, health care workers, and family members or friends who take care of elderly patients should still be vaccinated each year.