The flu shot usually contains the ingredients necessary to make our bodies start building up antibodies to the virus. But, viruses are notoriously shifty and can mutate in unexpected ways. This year, only about half of a sampling of recent flu cases matched the influenza A strain (H3N2) component in the shots right now. And, unfortunately, the CDC says it's now too late to get a new, more effective version out there for this season.
But, please do not use this as your excuse to not get a flu shot. Other virus strains haven't mutated, so the shot will still be effective against them. And, the vaccine will still be partially effective against the current H3N2 strain. Any protection is better than no protection, and remember — it's not just about protecting yourself. The rest of us will thank you.