Varieties of Influenza
Influenza is an extremely variable, fast-mutating virus. There are three main types, or genera: Influenzavirus A, Influenzavirus B, and Influenzavirus C. Though they are related, each has different disease characteristics. Of the three, influenza A mutates the fastest, and as a result, it is classified into number of subtypes. Influenza B evolves more slowly but still has many strains. Influenza C is the most stable of the three viruses.
Influenzavirus A causes the most severe form of influenza. It mutates 2-3 times faster than Influenzavirus B and is responsible for most large pandemics.
Influenzavirus A classification is based on variations on two proteins found on the virus surface. Hemagglutinin, abbreviated H, has 16 types, and neuraminidase abbreviated N, has nine subtypes. Theoretically, that means 144 subtypes are possible, although only a portion of these have actually been observed. The most significant subtypes to humans are H1N1, H1N2, and H3N2 viruses. The subtypes are sometimes written A/H1N1 or A (H1N1) to indicate that these are strains of influenza A.
At right: Transmission electron micrograph of the influenza virus. The surface proteins, hemagglutinin and neuraminidase, are clearly visible around the edges. Image from the CDC.
Subtype H1N1 is responsible for both the deadly 1918 Spanish flu pandemic and the 2009 swine flu outbreak. H5N1, H7N7, and H7N3 are a few of the subtypes associated with birds or avian flu, with H5N1 being the subtype currently being closely monitored as a potential pandemic avian flu.
Influenza A subtypes are further divided into strains that are named after the location and year of the outbreaks they cause. For example, the virus strain that caused a swine flu outbreak among soldiers in New Jersey in 1976 is called A/New Jersey/1976 (H1N1). The strain number may also be incorporated into the strain name. A person who is immune to a specific strain, either through vaccination or after being infected with it once, is not necessarily immune to other strains of the same subtypes because different strains may not be recognized by the immune system.
At left: Diagram showing the classification and naming Influenzavirus A subtypes and strains. Image by Tim Vickers (public domain).
Influenza B can cause illness and death in humans, but is not known for causing worldwide pandemics. It can spark epidemics, but these are generally less severe than those that Influenza A can cause. Influenzavirus B is usually included in the annual flu shot developed by health authorities, along with two strains of Influenzavirus A.
Influenzavirus B does not go through the kind of evolution that changes the virus surface proteins in Influenzavirus A, so it does not have subtypes. Influenza B does mutate, though more slowly than influenza A, and has different strains. As with influenza A strains, immunity to one influenza B strain does not necessarily mean a person is immune to other strains. An example of an Influenzavirus B strain is B/Hong Kong/330/2001.
Influenza C is the rarest and most stable (slowest evolving) of the three species of influenza. It usually causes mild illness, though it can occasionally cause more severe flu. Outbreaks are typically local and do not become large epidemics or global pandemics. An example of an Influenzavirus C strain is C/California/78.
“Influenza Viruses” from the CDC.