The A, B, and C of Influenza Virus

Via virology.ws -

Influenza A viruses tend to garner most of the attention, but let’s not forget that there are two other virus types, B and C.

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Influenza B viruses cause the same spectrum of disease as influenza A. However, influenza B viruses do not cause pandemics. This property may be a consequence of the limited host range of the virus – humans and seals – which limits the generation of new strains by reassortment. The virus causes significant morbidity: in the US in 2008, approximately one-third of all laboratory confirmed cases of influenza were caused by influenza B (as shown on the first graph on this CDC page). Consequently the seasonal trivalent influenza vaccine contains an influenza B virus component.

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Nearly all adults have been infected with influenza C virus, which causes mild upper respiratory tract illness. Lower respiratory tract complications are rare. There is no vaccine against influenza C virus.

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Each seasonal influenza vaccine contains three influenza viruses-one A (H3N2) virus, one regular seasonal A (H1N1) virus (not the 2009 H1N1 virus), and one B virus. The viruses in the vaccine change each year based on international surveillance and scientists' estimations about which types and strains of viruses will circulate in a given year.

This year's seasonal trivalent influenza vaccine in the US contains the following: A/Brisbane/59/2007 (H1N1); A/Brisbane/10/2007 (H3N2); B/Brisbane/60/2008.

The influenza vaccine composition used in the 2009-2010 influenza season in the U.S. is identical to that recommended by the World Health Organization on February 12, 2009, for the Northern Hemisphere's 2009-2010 influenza season.