Antigenic drift refers to mutations in the influenza virus over time. Such mutations occur almost yearly in the influenza virus. For this reason, vaccination is required on a yearly basis. In influenza mutations happen frequently because the virus is highly unstable and has no way of checking its DNA for errors. Even a tiny error in the DNA is permanent. Antigenic drift has been responsible for large outbreaks in the past, like the outbreak of influenza A Fujian(H3N2) in the 2003 - 2004 flu season . All influenza viruses experience some form of antigenic drift, but it's most pronounced in the influenza A virus. Antigenic drift is not the same as antigenic shift, which is a major change in the surface proteins on the virus.