Antigenic drift
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Antigenic drift refers to mutations in the influenza virus that cause changes in the virus's surface proteins over time. Those proteins (hemagglutinin and neuraminidase) are the causes of the body's immune reaction (i.e., they are antigens). Mutations occur almost yearly in the influenza virus, and while the change might not be a major one (which would then be called an antigenic shift), they are sufficient to lessen your body's ability to protect you with antibodies. For this reason, vaccination is required on a yearly basis, with the vaccine adjusted to the new antigen. In influenza, mutations happen frequently because the virus has no way of checking its RNA for errors. 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'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.