Gradual change in an influenza strain's antigens by point mutation is _________.

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Antigenic drift refers to the gradual changes in the influenza virus's antigens due to point mutations in the genes encoding these antigens. This process is a key factor in the annual seasonal variation of influenza, as it can lead to changes in the virus that make previous immunity (from past infections or vaccinations) less effective, necessitating yearly updates to the influenza vaccine. Antigenic shift, on the other hand, involves a more sudden and significant change in the virus's antigens, often leading to pandemic strains of influenza.

Gradual Change in an Influenza Strain's Antigens by Point Mutation