Question

Why is each year's vaccine different?

Answer 1

Flu vaccinations are needed each year to ensure you have continuous protection. Though you may have received a flu vaccine last year, your body's immune system from previous vaccinations gradually decreases over time. By the time flu season comes around, your body's immunity to it is virtuality non-existent.

Step-by-step explanation:

Answer 2

Because the birds that fly overseas carry different flu viruses every year.