When it comes to developing a seasonal flu vaccine, researchers must take aim at an invisible target.

Not only are flu viruses constantly changing, drug manufacturers need to start producing their vaccines four to six months before the flu season starts. That means scientists must predict which virus will be most active months before it even appears.

Like weather forecasters, sometimes they get it right and sometimes they get it wrong. This season, it looks like they got it wrong.

On Thursday, the director of the Centers for Disease Control and Prevention warned that the flu shots and nasal sprays distributed this year may not be a good match for the most active virus now circulating in the U.S.

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