Antigenic drift is the evolution of viruses caused by random genetic mutations. Antigenic drift and shift are the two mechanisms that cause antigens to change shape meaning they cannot be recognized by antibodies specific for them and an antibody-antigen complex cannot form  Antigenic drift is the most common and occurs every 2-3 years. This is because the genetic mutations result in small changes, producing viruses very similar to each other. Over time these changes accumulate leading to a virus that cannot be recognized by antibodies meaning the body has lost its immunity.
Antigenic drift can occur in all Influenza viruses and is caused by a point mutation of two proteins; hemagglutinin and neuraminidase. The genes usually altered are those that code for the epitope – the part of the antigen which antibodies bind to target a virus for destruction.
Antigenic drift is the reason why there is a new flu vaccine every year