There is no evidence that drinking in moderation is harmful after most vaccinations, but there is no absolute rule for this question. The first thing to determine is whether we are talking about a couple of drinks or excessive alcohol consumption.
It is considered sensible to avoid alcohol for several hours directly after your flu jab and if you must drink later on in the day, drinking moderately is advised.
In general, excessive alcohol is never advised as it is linked to many health concerns. Excessive alcohol drinking is known to suppress your immune system which will in turn reduce the amount of protection that the flu jab can offer.
In Summary: It’s definitely OK to drink after you get a flu jab so long as you drink in moderation.