Influenza virus antigen shift (antigenic shift) occurs by which mechanism and leads to what epidemiological consequence?

• A Point mutations in RNA polymerase generating new HA/NA variants — causes seasonal epidemics
• B Reassortment of segmented RNA genome segments between two different influenza A strains coinfecting the same host cell — can generate novel HA/NA combinations causing pandemic potential ✓
• C Recombination between influenza A and B strains — responsible for cross-reactive immunity
• D Insertion of retroviral sequences into the HA gene — allows immune evasion in immunocompromised patients

Explanation

Antigenic shift is an abrupt, major change in influenza A virus surface antigens (HA and/or NA) that occurs through genetic reassortment — the segmented RNA genome (8 segments) of two different influenza A strains can exchange segments when they co-infect the same cell (e.g., human + avian + swine strains in a pig 'mixing vessel'). This can generate a novel HA/NA subtype combination against which the human population has little or no pre-existing immunity, resulting in pandemic potential. Antigenic drift (point mutations by error-prone RNA polymerase) causes minor changes in HA/NA and is responsible for seasonal epidemics. Influenza A and B do not reassort. Retroviral insertion is not a mechanism.

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

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