The CDC announced yesterday that this year's flu has officially become an epidemic. One reason: the vaccine being used is not working.

This year's most dominant strain is the H3N2, a type of flu that causes more hospitalizations and death. The CDC warned in early December that this year's flu season would be particularly bad because the vaccine it built for this season isn't tough enough to fight against the H3N2 strain.

The problem was caused by a "drift," when the flu mutates to something else that the vaccine wasn't built to protect against. Although these kinds of mutations are common, this year's drift wasn't detected until it was too late to make a new vaccine.

Medical experts like Trish Perl, who heads up the Johns Hopkins Medicine Office of Epidemiology and Infection Prevention, have told the Post that "the most concerning thing about the flu season this year there is a mismatch between the predominant strain that is circulating and what was put in the vaccine."