Updated: Mar 6, 2021
The flu is caused by a virus, called the influenza virus. When we are infected by the flu virus, our immune system creates cells to fight and remember the virus so we cannot be infected by it again. The Influenza virus, much like the common cold virus, easily mutates and creates numerous strains. This means it changes proteins on itself so it can avoid our immune system from recognizing it and it can then infect us again the next year. This is called “antigenic drift” and is the main reason this vaccine is repeated each year.