Flu: This Year's Vaccine Is Worse Than We Thought

CDC says the vaccine is only 23 percent effective against flu.

"Scientists had to select strains for the vaccine back in February," said ABC News chief health and medical editor Dr. Richard Besser. "Between then and now the flu virus mutated significantly. Unfortunately, it is too late to make a change for this year. We are stuck with the vaccine we've got. CDC still recommends people get vaccinated since some protection is better than one."

The shot is even less effective among people over 50 years old, protecting against only 14 percent of the viruses, according to the new report.

People from 18 to 49 are only protected against 12 percent protection of them, but the vaccine offers children between 6 months and 17 years old 26 percent effectiveness. However, the age breakdowns are based on small samples and may change.

Besser explained that vaccines often work differently among different age groups because immune systems are "constantly changing" as people age.