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Immunology

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

Influenza type A and B viruses contain 8 genes that code for 10 proteins which includes the surface proteins haemagglutinin (HA) and neuraminidase (NA) (1). So far 16 HA subtypes and 9 NA subtypes have been identified.

Two antigenic changes are considered to be the hallmark of human influenza viruses:

  • antigenic drift
    • there is gradual and relatively continuous change in the viral HA and NA proteins caused by point mutations during viral replication
    • results in new virus strains in both type A and B viruses
    • emergence of these new strains causes
      • the need to frequently update the influenza virus vaccine strains
      • several influenza infections over a lifetime of an individual
  • antigenic shift
    • occurs in influenza type A viruses when either a HA protein or a combination of HA and NA proteins that have not been circulating among humans in recent years emerges
    • three mechanisms by which a new influenza virus may emerge:
      • genetic reassortment of non-human and human influenza viruses
      • an influenza virus from other animals (e.g. birds or pigs) can infect a human directly without undergoing genetic reassortment; or
      • a non-human virus may be passed from one type of animal (e.g. birds) through an intermediate animal host (such as a pig) to humans

Antigenic shifts occur infrequently and unpredictably while antigenic drift occurs continuously.

A large proportion (or even all) of the world’s population will be susceptible to new influenza viruses as a result of antigenic shift. If this new influenza virus has the capability of continuous human to human transmission leading to community-wide outbreaks, it may spread worldwide causing a pandemic (1).

Virus replication occurs in the epithelial cells of the respiratory tract.

Reference:

  1. World Health Organization (WHO). WHO Global Influenza Surveillance Network. Manual for the laboratory diagnosis and virological surveillance of influenza

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