Influenza classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Classification
Adapted from CDC[1] There are three types of influenza viruses: A, B, and C. Only influenza A viruses are further classified by subtype on the basis of the two main surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Influenza A subtypes and B viruses are further classified by strains.
Influenza Type A and Its Subtypes
- Influenza type A viruses can infect people, birds, pigs, horses, and other animals, but wild birds are the natural hosts for these viruses. Influenza type A viruses are divided into subtypes and named on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA).
- For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein; similarly an “H5N1” virus has an HA 5 protein and an NA 1 protein.
- There are 16 known HA subtypes and 9 known NA subtypes. Many different combinations of HA and NA proteins are possible. Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in general circulation among people.
- Other subtypes are found most commonly in other animal species. For example, H7N7 and H3N8 viruses cause illness in horses, and H3N8 also has recently been shown to cause illness in dogs.
- Only influenza A viruses infect birds, and al known subtypes of influenza A viruses can infect birds.
- However, there are substantial genetic differences between the influenza A subtypes that typicaly infect birds and those that infect both people and birds.
Three prominent subtypes of the avian influenza A viruses that are known to infect both birds and people are:
Influenza A H5
- Nine potential subtypes of H5 are known.
- H5 infections, such as HPAI H5N1 viruses currently circulating in Asia and Europe, have been documented among humans and sometimes cause severe illness or death.
Influenza A H7
- Nine potential subtypes of H7 are known. H7 infection in humans is rare but can occur among persons who have direct contact with infected birds. Symptoms may include conjunctivitis and/or upper respiratory symptoms.
- H7 viruses have been associated with both LPAI (e.g., H7N2, H7N7) and HPAI (e.g., H7N3, H7N7), and have caused mild to severe and fatal ilness in humans.
Influenza A H9
- Nine potential subtypes of H9 are known; influenza A H9 has rarely been reported to infect humans.
- However, this subtype has been documented only in a low pathogenic form.
Influenza Type B
- Influenza B viruses are usual y found only in humans.
- Unlike influenza A viruses, these viruses are not classified according to subtype.
- Influenza B viruses can cause morbidity and mortality among humans, in general are associated with less severe epidemics than influenza A viruses.
- Although influenza type B viruses can cause human epidemics, they have not caused pandemics.
Influenza Type C
- Influenza type C viruses cause mild illness in humans and do not cause epidemics or pandemics.
- These viruses are not classified according to subtype.
Strains
- Influenza B viruses and subtypes of influenza A virus are further characterized into strains.
- There are many different strains of influenza B viruses and of influenza A subtypes.
- New strains of influenza viruses appear and replace older strains; this process occurs through antigenic drift.
- When a new strain of human influenza virus emerges, antibody protection that may have developed after infection or vaccination with an older strain may not provide protection against the new strain. Therefore, the influenza vaccine is updated on a yearly basis to keep up with the changes in influenza viruses.