Propose a hypothesis of why the flu vaccine must be given every year but for most viral infections one or two doses of the vaccine provides lifelong immunity.

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The flu resulting due to influenza viruses get change due to a phenomenon termed as antigenic drift. These are the minor mutations or variations in the genes of the influenza virus, which can result in the modifications in the virus's surface proteins.  

It is because of the antigenic drift that an individual becomes vulnerable to infection of flu again, as antigenic drift has modified the virus to such an extent that the prevailing antibodies of the individual would not recognize and neutralize the newly formed influenza viruses.  

Thus, antigenic drift is the prime cause that why an individual gets the flu more than one time, and it is the main reason for reviewing the composition of the flu vaccine and is updated every year to overcome the infections caused by influenza viruses. This is why vaccinations of influenza virus are given every year.