"Gradual change in the antigens of influenza virus by mutation."

A gradual change of the hemagglutinin or neuraminidase proteins on the surface of a particular strain of influenza virus occurring in response to host antibodies in humans who have been exposed to it. It occurs on an ongoing basis in both type A and type B influenza strains and necessitates ongoing changes in influenza vaccines.

Many viruses mutate frequently in order to avoid destruction by the immune systems of their hosts. The gradual evolution of viral strains that results from these mutations is known as antigenic drift.