Yearly vaccination of humans for influenza viruses is necessary because

rapid mutation in flu viruses alters the surface proteins in infected host cells

Explanation:

Vaccines contain weakened or dead microorganism. Therefore, vaccination can be explained as the injection or introduction of weakened or dead microbe into the body, in order to stimulate the immune system against the particular microorganism, thereby preventing disease or infection.

In this case, the vaccination of humans for influenza viruses is to prevent the viruses from causing diseases in human. And this is done yearly , because the viruses undergo rapid mutation, this means that they change their genetic structure, and this will prevent the immune system in fighting the viruses. This is the reason why , the vaccination is given yearly so that, the immune system can cope with the changes.

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