There are three basic types of the influenza virus: A, B and C, according to the Centers for Disease Control and Prevention (CDC). Influenza A and B are the most common strains that afflict the estimated five million to 20 million Americans annually. Influenza C causes mild respiratory illness that probably goes unrecognized as the flu.
Influenza A
Influenza A viruses are divided into subtypes based on two antigens or proteins on the surface of the virus: hemagglutinin and neuraminidase. Hemagglutinin antigen (H) is a glycoprotein (protein with carbohydrate side chains) that allows the virus to bind to sialic acid and fuse with the host cell membrane. Neuramidase antigen (N), on the other hand, breaks down sialic acid, allowing the virus to disperse from the infected cell, once it has replicated inside of it.
According to "Harrison's Principles of Internal Medicine," there are 16 different H subtypes and nine different N subtypes, of which only H1, H2, H3, N1, and N2 have been associated with epidemics of disease in humans.
Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. Within these subtypes, there are also different strains of influenza virus. For example, although both H1N1 and H3N2 strains are included in the seasonal flu vaccine, in 2009 the Food and Drug Administration approved a separate vaccine for a new strain of H1N1 virus that was identified in the United States and Mexico in March 2009. This strain was different from previous human influenza A (H1N1) strains, and the separate vaccine was intended to thwart this new and, according to the World Health Organization, "pandemic" strain.
Influenza B
Influenza B, says the CDC, is not divided into subtypes. However, there are different strains of influenza B. Both influenza A and B frequently mutate resulting in new strains. This is why the flu vaccine changes every year. According to a 2005 report in the CDC's Advisory Committee on Immunization Practices, influenza B viruses mutate less frequently than influenza A viruses and cause less severe disease.
Influenza C
Influenza C, like influenza B, has no subtypes. It mutates even less frequently than influenza B and perhaps not at all. Influenza C does not cause significant human disease. It has never been linked to an epidemic and influenza C strains are not included in the seasonal flu vaccine.
References
- CDC: Seasonal Influenza -- Types of Influenza Viruses
- "Harrison's Principles of Internal Medicine, 17th edition" ; A.S. Fauci, E. Braunwald, et. al, . 2008
- Prevention and Control of Influenza
