The reason for that is that the influenza virus has a rapid rate of mutation, forcing companies that manufacture the vaccine to make their best guess as to which flu types will be more prevalent in a given season.

FORBES: One Step Closer to a Universal Flu Vaccine?

This year's North American vaccine is made from three viruses: two types of influenza A virus (H3N2 and H1N1) and an influenza B virus.

CNN: Had a flu shot? You'll be OK -- maybe

As of January 5, the CDC said Friday, the predominant virus in the United States was an influenza A (H3N2) virus that matched well with the H3N2 virus represented in the vaccine.

CNN: Had a flu shot? You'll be OK -- maybe