Antigenic drift

Antigenic drift is the process of random accumulation of mutations in viral genes recognized by the immune system. Such accumulation may significantly change the antigens of the virus, and may help it evade the immune system. This process may lead to a loss of immunity, or in vaccine mismatch when one of the strains selected for the vaccine doesn't optimally match the circulating strains. Antigenic drift may also allow a virus to jump to a new host species.

Antigenic drift in Influenza Viruses
In the influenza virus, the two relevant genes are the surface proteins, hemagglutinin and neuraminidase. The hemagglutinin is responsible for entry into host epithelial cells while the neuraminidase is involved in the process of new virions budding out of host cells. The host immune response to viral infection is largely determined by the immune system's recognition of these influenza antigens. Vaccine mismatch is a potentially serious problem. Antigenic Drift is continuous process of genetic change among flu strains.

As in all RNA viruses, mutations in influenza occur frequently because the virus' RNA polymerase has no proofreading mechanism, providing a strong source of mutations. Mutations in the surface proteins allow the virus to elude some host immunity, and the numbers and locations of these mutations that confer the greatest amount of immune escape has been an important topic of study for over a decade.

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 should not be confused with antigenic shift, which refers to a more abrupt change in the antigenes. As well, it is different from random genetic drift which is a very different but important process in population genetics.