Antigenic drift is a process where small genetic changes occur in the viruses over time, leading to slight alterations in proteins on their surface. A common example is the annual mutation in the influenza virus. It is caused by error-prone replication of the viral genome which can affect vaccine effectiveness by reducing their ability to provoke immunity. Antigenic shift, unlike drift, is a sudden significant change, often due to gene reassortment, leading to a new subtype of virus.