Antigenic drift
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 it's
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.