Focus on the Flu

Antigenic Drift vs. Antigenic Shift

Antigenic Shift

The genetic change that enables a flu strain to jump from one animal species to another, including humans, is called antigenic shift.  Antigenic shift can happen in three ways:

Antigenic Shift 1

• A duck or other aquatic bird passes a bird strain of influenza A to an intermediate host such as a chicken or pig.
• A person passes a human strain of influenza A to the same chicken or pig.
• When the viruses infect the same **ll, the genes from the bird strain mix with genes from the human strain to yield a new strain.
• The new strain can spread from the intermediate host to humans.

Antigenic Shift 2

• Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans.

Antigenic Shift 3

• Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to an intermediate animal host and then to humans.

The new strain may further evolve to spread from person to person. If so, a flu pandemic could arise.