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I Got the Flu Shot! Why Did I Get the Flu?

We are well within the cold and flu season.  Many people especially the elderly and the immunocompromised have received their flu shots.  Despite the efforts to control the flu we still see many people who get the flu.  Why?

The flu is caused by influenza virus and belongs to a family of viruses called Orthomyxoviridae.  There are three common subtypes: Influenza A, B and C.  The most virulent type that causes most severe disease is Influenza A.  Viruses need a host cell to survive.  They have a simple structure containing genetic material enclosed in membrane called a capsid.  The Flu virus has another membrane called an envelope surrounding it.  On the surface of the envelope there are substances called glycoproteins.  These glycoproteins are named Hemagglutinins (HA) and neuraminidase (N).  HA on the surface of the virus
allows for the entry to the host cell.  N is used for the release of new viruses from the host cell.  There are 16 known subtypes of HA and 9 known subtypes of N.  HA and N can form many combinations.  A recent example was in 2009 H1N1 epidemic.

Because there are so many combinations, the flu vaccine cannot vaccinate against all subtypes.  The flu shot (vaccine) is based on the most common strains and developed from common trends and data.  It is possible despite having a flu shot to get the flu.  Following good sanitation practices and handwashing is
our only defense.


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