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

— -- This year's flu vaccine is only 23 percent effective against this year's predominant strain, H3N2, according to a new report out today from the Centers for Disease Control and Prevention.

In the middle of a bad flu season, it turns out the vaccine doesn't offer much protection against the flu because the predominant strain is H3N2 because it has mutated since the vaccine was created and manufactured, according to the CDC.

"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.

"One of the reasons they developed a high dose flu vaccine for the elderly was because the elderly never respond as well as other age groups, and their risk of having severe flu is the greatest," he said.