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Types of Influenza Viruses
There are three types of influenza viruses: A, B and C. Influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. 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. The current subtypes of influenza A viruses found in people are A (H1N1) and A (H3N2).
Influenza B viruses are not divided into subtypes. Influenza B viruses also can be further broken down into different strains.
Influenza A (H1N1), A (H3N2), and influenza B strains are included in each year's influenza vaccine. Getting a flu vaccine can protect against influenza A and B viruses. The flu vaccine does not protect against influenza C viruses.
Related information on Influenza A(H1N2) can be found at:
- Page last modified December 20, 2007
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
