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How the Flu Virus Can Change: "Drift" and "Shift"
Influenza viruses can change in two different ways.
One is called "antigenic drift." These are small changes in the virus
that happen continually over time. Antigenic drift produces new virus strains
that may not be recognized by the body's immune system. This process works
as follows: a person infected with a particular flu virus strain develops antibody
against that virus. As newer virus strains appear, the antibodies against the
older strains no longer recognize the "newer" virus, and reinfection
can occur. This is one of the main reasons why people can get the flu more
than one time. In most years, one or two of the three virus strains in the
influenza vaccine are updated to keep up with the changes in the circulating
flu viruses. So, people who want to be protected from flu need to get a flu
shot every year.
The other type of change is called "antigenic shift." Antigenic shift
is an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin
and/or new hemagglutinin and neuraminidase proteins in influenza viruses that
infect humans. Shift results in a new influenza A subtype. When shift happens,
most people have little or no protection against the new virus. While influenza
viruses are changing by antigenic drift all the time, antigenic shift happens
only occasionally. Type A viruses undergo both kinds of changes; influenza
type B viruses change only by the more gradual process of antigenic drift.
- Page last modified December 17, 2007
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
