You are correct. I got zapped by flu in November and it was gruesome. Never even had a cold in 4 years. I had a flu shot in October, but obviously did not help. Now it is January, and I am still feeling weak, and have occasional cough. I got it from my wife. She got it from her office co-workers or patients.
I have never had the flu, & never had a flu shot, nor any other immune system destroyer, since I was a kid. The only people I know, personally, who had the flu, have also had a flu shot, the previous month or three, before. Just “food for thought,” entropy