[h=1]Types of Influenza Viruses[/h]There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza virus to infect people can cause an
influenza pandemic
. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus (
CDC 2009 H1N1 Flu website) emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus caused the first influenza pandemic in more than 40 years. That virus (often called “2009 H1N1”) has now mostly replaced the H1N1 virus that was previously circulating in humans.
Influenza B viruses are not divided into subtypes, but can be further broken down into different strains
This is a pretty easy to understand paragraph from the Center for Disease Control that makes it easy to understand why you can get still get flu symptoms after having the shot. The shot only covers the strains of flu that are most likely to cause epidemics, not the mild seasonal flu strains. Those we are expected to deal with.