Each year’s flu vaccine contains three flu strains—two A strains and one B strain—that can change from year to year.
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.
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Last Updated January 14, 2011