Structure of the influenza virus:
The genetic material (RNA) that is responsible for the multiplication of the viruses is enveloped by the virus membrane, an envelope made up of lipids. In this envelope there are proteins such as haemagglutinin (H) or neuroaminidase (N), so-called glycoprotein spikes. They are responsible for the binding to the host cell and the penetration of the RNA into the cell, as a result of which the virus can then multiply in the host cell. These proteins exactly match the surface structures of the preferred host cells. This explains the host specificity for e.g. birds, swine or humans, i.e. why a particular animal species or human can become infected or not.
At the same time, the glycoproteins are also the antigens against which the immune system of the host (animal or human) forms antibodies. Vaccines also contain these antigens and lead after vaccination to the formation of antibodies.
Epidemiology
The influenza viruses can alter very rapidly in their properties (point mutation, antigen drift). As a result, there are repeatedly new “flu waves” or flu epidemics. Accordingly, vaccines also have to be repeatedly adapted to these changes in order to be effective.
The most feared change is that which can occur through the fusion of e.g. a swine flu virus with a human flu virus during simultaneous infection of a host cell (antigen shift). This can result in viruses that spread particularly rapidly, cause severe illness and lead to the feared pandemics.
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