Immunology

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