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 cell, 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.
Credit: This image is in the public domain. Please credit the National Institute of Allergy and Infectious Diseases (NIAID).
Illustrator: Links Studio.
Download: high resolution version of the Antigenic Shift illustration.

Antigenic Drift

Each year’s flu vaccine contains three flu strains -- two A strains and one B strain -- that can change from year to year.

1.After vaccination, your body produces infection-fighting antibodies against the three flu strains in the vaccine
2.If you are exposed to any of the three flu strains during the flu season, the antibodies will latch onto the virus’s HA antigens, preventing the flu virus from attaching to healthy cells and infecting them.
3.Influenza virus genes, made of RNA, are more prone to mutations than genes made of DNA.
4.If the HA gene changes, so can the antigen that it encodes, causing it to change shape
5.If the HA antigen changes shape, antibodies that normally would match up to it no longer can, allowing the newly mutated virus to infect the body’s cells. This type of genetic mutation is called “antigenic drift.”

Credit: This image is in the public domain. Please credit the National Institute of Allergy and Infectious Diseases (NIAID).
Illustrator: Links Studio.
Download: high resolution version of the Antigenic Drift illustration.