Influenza virus
Under an electron microscope, influenza viruses look like small spheres (100 to 150 nm) with a distinctly visible membrane and numerous spikes.
Spikes are particles of the receptor-binding protein called hemagglutinin (HA). The external part of this protein is responsible for binding to the respective cellular receptors.
Numerous supplementary structures are presented in the schematic drawing of the virus particle. In addition to hemagglutinin, also two other proteins are anchored in the membrane: the neuraminidase (NA) enzyme and protein M2 playing the role of an ion channel between the environment and the interior of the virus.
The membrane consists of fat particles. The viral capside consisting of the layer formed by protein M1 particles is found within the membrane. The capside envelops the viral interior. The genetic material of the virus is eight single parts (segments) of RNA. Each RNA segment is combined with an enzyme (polymerase) and nucleoprotein. The whole complex is called ribonucleoprotein (RNP).
Types and subtypes of influenza virus.
Three types of influenza virus: A, B and C, are distinguished on the basis of the antigenic properties of the core proteins (M1, nucleoprotein).
The vaccine contains also antigens of type A and type B influenza virus.
Type A
Type A influenza virus is found in mammals (e.g. human, horse, pigs) but its largest reservoir is aqueous birds throughout the world: ducks, geese, cormorants and many other species. Usually, there is a strict limitation of virus occurrence with respect to the host; human influenza virus type A with difficulty infects geese and vice versa. Only pigs are infected equally easily with both human as well as avian strains of type A virus.
Virus type A is responsible for epidemics and pandemics among humans.
Type B
Influenza virus type B was discovered only in humans.
Type C
Influenza virus type C occurs ubiquitously but does not give disease symptoms and is of lower significance in human infections. Type C virus antigens are not included in the influenza vaccine.
Subtypes of influenza virus
Only type A virus has subtypes. Classification into subtypes is based on hemagglutinin (H) and neuraminidase (N), and indicates antigenic features of membrane proteins of type A influenza virus.
So far, 16 subtypes of hemagglutinin (H1- H16) and 9 subtypes of neuraminidase (N1-N9) have been isolated. H1, H2, H3 and N1, N2 subtypes are characteristic for the human virus. Nevertheless, the virus will may undergo antigenic shift as a result of mutation and infection of a human with the subtype normally infectious only for other species will be possible.
Aquatic birds are a reservoir of all subtypes of hemagglutinin and neuraminidase.
Designation of influenza viruses
When the virus strain is isolated and identified, it acquires an English description defining its type, the host, the subsequent number and the isolation year (and subtype for type A virus). For example, strain:
A/human/Philippines/2/82 (H3N2) is a type A virus isolated from a human on Philippines in 1982,
A/equine/Prague/1/56 (H7N7) is a type A virus isolated from a horse in Prague in 1956, and B/Beijing/184/93 is a type B virus isolated from a human in the capital of China in 1993.
H and N refer to hemagglutinin and neuraminidase, respectively, and indicate, only in the case of type A virus, the subtype consistently with certain antigenic features of their membrane proteins.
Sometimes, the influenza virus has the suffix “-like”, e.g. B/Harbin/7/94, B/Beijing/184/93-like (virus B/Harbin/7/94 similar to strain B/Beijing/184/93). This applies to strains which are genetically closely related or even indistinguishable, which differ only by the isolation place or time. In this example, the strains were isolated in Beijing in 1993 and in Harbin in 1994, but both strains proved to be very similar. This nomenclature is often used in recommendations for vaccine composition in the specific year.
