The flu vaccine changes every year. The flu vaccine is based on a predicted strain that is anticipated to be the most infectious that year. Thats why people who sometimes get the vaccination still get the flu, they don't have the antibodies for the strain they were infected with. Strains for viral disease don't change very much and usually the vaccination for them cover them well.
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.
