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Why do changes in influenza surface antigens make new outbreaks possible?
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influenza
A colored transmission electron micrograph showing influenza viruses (red) at the outer surface of a host cell.
Changes in influenza surface antigens—specifically the glycoproteins hemagglutinin and neuraminidase—enable new outbreaks by allowing the virus to evade the human immune system. Because these proteins serve as the primary targets for antibodies, modifications to their structure mean that existing immune defenses may no longer recognize or neutralize the virus.1234
These changes occur through two primary mechanisms:
- Antigenic drift: The gradual accumulation of random genetic mutations that cause minor changes to the antigens. This process allows the virus to evolve into new strains that can bypass partial population immunity.24
- Antigenic shift: A sudden, major genetic alteration, often resulting from genetic reassortment when a host is infected by multiple influenza strains. This creates a new subtype with novel antigens to which humans have little to no pre-existing immunity, significantly increasing the risk of epidemics or pandemics.14
Because the virus is constantly evolving, annual vaccination is essential to provide protection against the most current circulating strains.5