Why does the influenza vaccine require annual reformulation, even with an adjuvant enhancing initial immunogenicity?

💡 Explanation

The influenza virus mutates rapidly via antigenic drift, generating new strains. This necessitates annual vaccine updates because the pre-existing B cell and antibody memory, even when boosted by an adjuvant, no longer effectively recognizes the altered viral antigens; therefore, the reformulated vaccine triggers a new immune response rather than relying on outdated immunity.

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