Learn how immunizing a critical portion of a community protects most members of the community.
Volunteer for NIAID-funded clinical studies related to influenza on ClinicalTrials.gov.
Confirmed human cases of avian influenza A (H5N1)
Timeline of human and animal H5N1 infections since 1996 (PDF)
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.
View Antigenic Shift
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Last Updated January 14, 2011