My last flu shot was in 1977, got healthy and the same week missed three day of work with the flu.
I have not had the flu since; there is some thing going around that I call Bronchitis because I had it and it acted like Bronchitis which up to age 13 I had every darn year.
No headache, not hard to breath, no running nose, but coughing up stuff for days.
It is odd when you are coughing that crap up continually but you have zero problems taking a deep breath.
I did find out when you got that stuff, going outside in sub-zero weather gives you a short coughing fit, then it goes away.
It is gone now.