If a novel influenza strain appears and spreads globally, why does prior vaccination against a different strain sometimes offer limited protection?

💡 Explanation

Limited protection arises because while vaccination induces antibody production, a novel strain may possess significantly different surface proteins. Consequently, the existing antibodies' affinity for the new epitopes is too low for effective neutralization; therefore, the virus can still infect cells, rather than being neutralized by the less-effective antibodies.

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