To inform selection of the vaccine viruses used in the seasonal influenza vaccine, Centers for Disease Control and Prevention (CDC) scientists use a test called the hemagglutinin inhibition (HI) assay to characterize influenza viruses according to the “antigens” on the virus’ surface. Antigens are molecular structures on the surface of viruses that are recognized by the immune system, and are capable of triggering an immune response, and antibody production. Antigens play a key role in the influenza virus’ ability to bind to, and enter cells in the nose, throat and lungs. The HI test measures how well antibodies made from an immune response against a specific influenza virus bind to other influenza viruses, and thus inactivate them. Scientists also use the HI test to compare antigenic changes in currently circulating influenza viruses with influenza viruses that have circulated in the past. Here, an HI machine is preparing to perform the HI test.
Influenza (flu) viruses change constantly. As a WHO Collaborating Center for Influenza (WHO CC) and the U.S. National Influenza Center, CDC monitors flu activity nationally and globally with other labs, looking for changes in circulating viruses. CDC monitors flu viruses because changes can impact the effectiveness of flu vaccine. When circulating viruses are substantially different from those in the vaccine, vaccine effectiveness can be reduced. If it looks like viruses are starting to change in specific ways (which can impact how well the vaccine works), this can trigger health authorities to recommend different viruses for vaccine production.
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