Final answer:
Antigenic drift involves small changes in spike proteins due to mutations, enabling the flu to reoccur. Antigenic shift causes major spike protein changes due to reassortment, potentially leading to flu pandemics.
Explanation:
Antigenic drift in influenza viruses is caused by small changes in spike proteins due to point mutations, while antigenic shift results in major changes in spike proteins due to gene reassortment from different viruses infecting the same host. Antigenic drift allows for the reoccurrence of the flu by creating new strains that evade existing immunity. Conversely, antigenic shift can cause a flu pandemic by creating novel virus strains for which the population has little to no pre-existing immunity, leading to widespread infections and severe outbreaks.
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