you have to get the flu shot every year or it's useless, I thought this was common knowledge.
Flu evolves at blazing speed, that's why every year they get all kinds of flu they can from Australia (or whatever is the first place where it arrives) and then sell the shots in Europe before it arrives.
If you don't do every year, it will work 100% just for the year you've done it.
I used to stay ill 1 week with flu and vomiting and diarrhea every year.
I started taking the flu shot every October and I've never been ill ever since, just a cold occasionally (max 1-2 times per winter).
If you rarely get the flu then don't bother ofc, unless flu puts you at risk of complications (elderly etc.).