How Influenza Viruses Change: Drift and Shift

Influenza viruses can change in two different ways:

Antigenic drift -

this type of change occurs through small changes in the virus that happen continually over time. Antigenic drift produces new virus strains that may not be recognised by antibodies to earlier influenza strains. This process works as follows: a person infected with a particular flu virus strain develops antibodies against that virus. As newer virus strains appear, the antibodies against the older strains no longer recognise the "newer" virus, and infection with a new strain can occur. This is one of the main reasons why people can get the flu more than once. 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. For this reason, people who want to be immunised against influenza need to receive a flu vaccination every year.


Antigenic shift -

this type of change is an abrupt, major change in the influenza A viruses, resulting in a new influenza virus that can infect humans and has a hemagglutinin protein or hemagglutinin and neuraminidase protein combination that has not been seen in humans for many years. Antigenic shift results in a new influenza A subtype. If a new subtype of influenza A virus is introduced into the human population most people will have little or no protection against the new virus and if the virus can spread easily from person to person, a pandemic (worldwide spread) may occur (see Influenza Pandemics). Influenza viruses are changing by antigenic drift all the time, but antigenic shift happens only occasionally. Influenza type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift