They won't stop you from getting the flu but they will lessen the effects if you do get it. With the COVID-19 pandemic complicating things, I think it is wise to get one. It's cheap insurance.
I got mine Friday at my local Kroger Pharmacy. This year they are immunizing against 4 strains of virus vs 3 in previous years. So you may get a little more tenderness in the arm. I did for about 12 hours.
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One infection raises the likelihood of a second "superinfection" on top of it. I got mine ASAP.