Last edited 02/2022

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.


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