Why the CDC is warning that this flu season is on track to be one of the worst in a decade 03:26. This season's influenza vaccine may have been a poor match to a strain of the flu virus that ...

Flu vaccine effectiveness in South America was 35% against hospitalization. ... A CDC spokesperson said that the effectiveness of the vaccine for H1N1 was "within the range" of previous seasons.

Two types of influenza viruses are responsible for seasonal flu outbreaks: Influenza A and B. Influenza A accounts for around 75 percent of all cases and tends to cause more severe illness in adults.

The flu vaccine, for everyone ages 6 months and older, is crucial, Dr. Eduardo Azziz-Baumgartner, a medical epidemiologist in CDC’s Influenza Division, previously told USA TODAY.