More than three-quarters of this year's flu shots were ineffective

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An interim report from the Centers for Disease Control and Prevention, released Thursday, found that this year's flu shot was only 23 percent effective overall. That means that more than 75 percent of Americans who got the flu shot could still become infected with the virus.

The discrepancy is due to "a mismatch between the flu strains covered by the shot and those actually causing illness in people," U.S. officials told Reuters. According to the CDC, this year's predominant flu virus, influenza A (H3N2), has "drifted" since the flu shot was created.

Reuters notes that age plays a large part in the flu shot's effectiveness — it tends to work better in young, healthy people and is least effective in protecting the elderly.