Peanuts contain a unique kind of lectin, which, unlike almost all other plant lectins, does not break down with preparation (heat, mechanical, etc.). Lectins irritate the gut lining and therefore cause gastro issues for a lot of people, peanuts probably moreso than other foods.

Do you get a flu shot every year?

I used to, but haven’t been regular about it for several years. Is there a link between it and adult onset allergies?

It has been proposed for at least egg allergy, since the flu shot contains egg protein and there is a plausible MoA. Some of the shots also use adjuvant 65, which contains peanut oil and it's possible the same MoA applies. Your own personal risk-reward calculation applies of course

> My symptoms are just gastro related, so it’s probably not the same as childhood peanut allergies, though.

Maybe it's food intolerance, which is distinct from food allergy? Those I'm sure can develop later in life.