Influenza virus undergoes 'antigenic shift.' Which molecular event specifically constitutes antigenic shift, and why does it cause pandemic influenza rather than seasonal epidemics?

• A Reassortment of segmented genome segments between human and animal influenza A strains in a mixed-infected host, producing a novel HA or NA subtype to which humans have no pre-existing immunity ✓
• B Point mutations in the haemagglutinin gene causing gradual changes in surface antigens year by year
• C Recombination between influenza A and influenza B viruses creating a new chimeric haemagglutinin
• D Mutation in the M2 ion channel gene causing amantadine resistance and new virulence

Explanation

Antigenic shift (major change) is unique to influenza A due to its 8-segmented RNA genome: when two different influenza A subtypes co-infect the same cell (usually swine as a 'mixing vessel'), genome segments reassort randomly, potentially generating a progeny virus with a completely new HA or NA subtype. Since most humans lack immunity to the new subtype, pandemic spread occurs. Antigenic drift (gradual point mutations in HA/NA by error-prone RNA polymerase) is responsible for seasonal epidemics, not pandemics. Influenza B does not undergo major reassortment with animal strains.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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