Two main mechanisms are identified14-17 for the changes in the influenza viruses, allowing for the recombination of antigens according to different H and N proteins: Antigenic Drift and Antigenic Shift. These comprise the changes in influenza virus genetic compositions that allow for influenza re-infection.
Antigenic Drift
This mechanism occurs within virus replication, producing small changes in the virus that originate virus with similar antigenic combinations. The accumulation of changes due to the antigenic drift may lead to the appearance of viruses with different antigenic combinations.
Antigenic Shift
This mechanism assumes a more sudden change in the influenza viruses antigen combination, for example through zoonosis. This abrupt change composes a major threat for pandemic influenza outbreaks since the population’s immunologic response may be totally unprepared to deal with the new antigenic combination of the influenza virus.