Reassortment can result in antigenic shift when an intermediate host, such as a pig, is simultaneously infected with a human and an avian influenza A virus.

What is antigenic drift caused by?

Antigenic drift: A subtle change in the surface glycoprotein (either hemagglutinin or neuraminidase) caused by a point mutation or deletion in the viral gene. This results in a new strain that requires yearly reformulation of the seasonal influenza vaccine.

What causes antigenic shift and drift?

Antigenic Shift

This shift typically occurs when a human flu virus crosses with a flu virus that usually affects animals (such as birds or pigs). When the viruses mutate, they shift to create a new subtype that is different from any seen in humans before. A human flu virus infects an animal, such as a pig.