Your Flu Shot Might Not Work

CDC just dropped an eye-catching warning: This year's flu vaccine might not fight the flu all that well.

And it's also too late to make a new vaccine, CDC director Thomas Frieden said on Thursday.

Why isn't the current flu vaccine as effective as researchers were hoping? Because the flu has mutated.

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As Maggie Fox points out at NBCNews.com, the current flu vaccine is designed to protect against several strains of flu, given that there's always a mix. And the flu regularly mutates, or 'drifts,' as it moves through the population.

But this year's predominant strain of flu, called influenza A (H3N2), began to mutate as early as March, when vaccine production was underway. Researchers were struck by H3N2′s new and different appearance, Frieden noted. 'This is not something that's been around before,' he said.

As a result, just half of current flu cases match the available vaccine. Given that it takes four months to make a new vaccine, health officials say that it's not practical to try and begin production of another vaccine.