Antigenic drift
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- Antigenic drift is distinct from antigenic shift, which is a faster mode of genetic change in viruses.
Antigenic drift is a mutation in the genetic makeup of the influenza virus that changes the virus's surface proteins over time. Those proteins (hemagglutinin and neuraminidase) cause a body's immune reaction to the virus (that is, they are antigens).
Mutations occur almost yearly in the influenza virus. Influenza mutations occur frequently because the virus has no way of checking its RNA for errors and RNA is less stable than DNA. While a mutation might be minor, it is sufficient to lessen the protection of the body by antibodies. Therefore, to be most effective, vaccination should be done every year, with a vaccine adjusted to the new antigen.
Antigenic drift has been responsible for heavier-than-normal flu seasons 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 is most pronounced in the influenza A virus.
Antigenic drift is not the same as antigenic shift, which is the process by which two different strains of influenza combine to form a new subtype having a mixture of the surface antigens of the two original strains.