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A, B, C, D: The Different Influenza Types

January 29, 2025 at 03:20 PM EST
By WeatherBug's Erik Gates and James West
A Flu vaccine diagram (CDC)

Millions of Americans are afflicted by the flu each year in the U.S., but did you know that only two main types of Influenza are responsible? 

Influenza viruses are divided into 4 major types, A, B, C and D. Seasonal outbreaks of the flu are the result of Influenzas A and B exclusively and only type A can produce global pandemics such as the 1918 Spanish Flu. Type C infections are rare and only cause minor illness in humans; they are no threat to produce epidemics. Cattle are the primary victims of type D influenza, and human illness has never been reported.

Subtypes of Influenza are classified based upon proteins on the surface of the virus. The names we often see in the media such as H1N1 or H5N1, are derived from the subtypes of two major proteins, hemagglutinin and neuraminidase, H and N. There can be 18 different hemagglutinin subtypes and 11 neuraminidase, or 198 possible combinations. The two main circulating subtypes of Influenza A are A(H1N1) and A(H3N2). 

These subtypes can then be broken down into groups and sub-groups, also known as clades and sub-clades. The yearly variation in virulence of a specific strain of the flu can be attributed to genetic changes at the group level. Though it is important to note that just because two different clades of a strain exist, doesn’t mean that one will always be more pathogenic than another. 

Type B virus is not broken down into subtypes, but rather two lineages, B/Yamagata and B/Victoria. These lineages can be similarly broken down into groups and sub-groups, but there is much less variation in the genetic properties of type B Influenza over time, resulting in its less virulent nature.

The flu vaccine administered each year before flu season protects against one influenza A(H1N1) strain and one influenza A(H3N2) strain, as well as one or two influenzas B viruses. A combination of data from both real-time infection rate and a laboratory setting allows the World Health Organization to select the specific strains of virus they wish to include in the seasonal flu virus. Remember, the flu vaccine is a very important step in preventing the spread of influenza. The U.S. Centers for Disease Control and Prevention estimated that in the 2017-2018 flu season alone, routine vaccination prevented an estimated 7.1 million influenza illnesses in the U.S.

Source: Centers For Disease Control and Prevention

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