This is anecdotal experience of course, but the one year I had a flu shot was also the year I had the worst flu of my life. So I am... ambivalent about the efficacy of those shots
Never take the damn things anymore. I trust my grandmother's advice: hot
rooibos, shot of whiskey, fresh lemon juice and a teaspoon or two of honey every night. Dunno about the flu, but it does wonders for me