This season's flu shot is only 36 percent effective. And that's up from 25 percent earlier this season, according to the Centers for Disease Control and Prevention.
That's contributing to a tough flu season, with effects not seen since the 2009 swine flu outbreak.
Creating an effective flu vaccine comes with challenges. The Food and Drug Administration said Thursday the viruses "can change their genetic make-up rapidly during the course of a single year."
The CDC said the vaccine works better on children than it expected, with an effective rate of 51 percent against one type of the flu virus.
Additional reporting from Newsy affiliate CNN.
