Influenza classification

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For more information about non-human (variant) influenza viruses that may be transmitted to humans, see Zoonotic influenza

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [3]


Influenza virus can be classified into 3 main types: A, B and C. Based on the surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), influenza type A is further classified into several subtypes and can infect many species, including humans. Type B can only infect humans and type C is not common and causes only mild illness.


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.[1]

Different hemagglutinin and neuraminidase subtypes and the species in which they have been detected
Species Hemagglutinin
Humans H1, H2, H3, H5, H6, H7, H9, H10 N1, N2, N6, N7, N8, N9
Poultry H1, H2, H3, H4, H5, H6, H7, H8, H9, H10, H11, H12, H13, H14, H15, H16 N1, N2, N3, N4, N5, N6, N7, N8, N9
Pigs H1, H2, H3, H4, H5, H9 N1, N2
Bats H17, H18 N10, N11
Adapted from CDC [2]

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 identified by 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 humans.
  • 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 A H5, A H7, and A H9.

Influenza A H5

  • Nine potential subtypes of H5 are known (H5N1, H5N2, H5N3, H5N4, H5N5, H5N6, H5N7, H5N8, and H5N9).
  • Most H5 viruses identified worldwide in wild birds and poultry are LPAI (low pathogenic avian influenza) viruses.
  • Sporadic H5 virus infection of humans, such as with highly pathogenic avian influenza A (H5N1) viruses currently circulating among poultry in Asia and the Middle East have been reported in 15 countries, often resulting in severe pneumonia with approximately 60% mortality worldwide.

Influenza A H7

  • Nine potential subtypes of H7 are known (H7N1, H7N2, H7N3, H7N4, H7N5, H7N6, H7N7, H7N8, and H7N9).
  • Most H7 viruses identified worldwide in wild birds and poultry are LPAI viruses.
  • H7 virus infection in humans is uncommon, but has been documented in persons who have direct contact with infected birds, especially during outbreaks of H7 virus among poultry. Illness in humans may include conjunctivitis and/or upper respiratory tract symptoms.
  • In humans, LPAI (H7N2, H7N3, H7N7) virus infections have caused mild to moderate illness.
  • HPAI (high pathogenic avian influenza) virus infections, subtypes H7N3 and H7N7, have caused mild to severe and fatal illness.

Influenza A H9

  • Nine potential subtypes of H9 are known (H9N1, H9N2, H9N3, H9N4, H9N5, H9N6, H9N7, H9N8, and H9N9)
  • All H9 viruses identified worldwide in wild birds and poultry are LPAI viruses.
  • H9N2 virus has been detected in bird populations in Asia, Europe, the Middle East and Africa.
  • Rare, sporadic H9N2 virus infections of humans have been reported to cause generally mild upper respiratory tract illness.

Influenza Type B

  • Influenza B viruses are usually 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.


  • 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.


  1. "CDC Influenza Viruses".
  2. "CDC Seasonal Influenza - Transmission of Influenza Viruses from Animals to People".

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