How many people have received the vaccine and how many people have contracted the flu.
The flu shot is 70 to 90 percent effective in those under 65. That rate drops to as low as 50 percent as you get above 65.
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However, those over 65 should still get the shot if they aren't allergic to ggs, because they are an at risk group (disproportionately killed by the flu in a normal flu season.)