Researchers at the University of Minnesota discovered that the most commonly prescribed flu vaccine in the U.S. is effective in just 59 percent of people between the ages of 18 to 65.
They also found the FluMist vaccine is 83 percent effective in children under the age of seven.
"Every season the flu strain that comes through our community changes, and the flu vaccine makers have to guess before the flu season what they think that will be. Some years they guess well and it works quite well, other years, not so well." said Dr. Richard Besser, ABC's medical editor.
Dr. Besser still recommends the flu vaccine, especially for the elderly, pregnant women and people with certain medical conditions.