Memory cells are the basis for vaccine effectiveness, with many vaccines provoking lifelong immunity from only a few rounds of vaccination. Why then do you need a new flu vaccine every year?-The flu vaccine is only weakly immunogenic and requires multiple shots for it to take effect-New strains of flu virus arise rapidly; this year's strain may be antigenically different from last year's-The immune memory for flu is short-lived-The flu virus evolves resistance to antibiotics

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