The reason why a flu vaccine is needed every year is because influenza mutates very frequently with multitude of strands different from each other. The vaccine is actually a mix of different strands most probable to be seen during the season.
Some years they have it right and the flu season is markedly mild for those who did get the vaccine some years they miss the dominant strand like last year. That's why you can still get the flu despite getting the vaccine, sometimes you just get lucky and don't get the flu even without a vaccine (maybe you were immune to that particular strand).
https://www.livescience.com/flu-shot...fluenza-b.html
When people were claiming to have had a major illness in late November and December on forums, yes it could have been covid, but it most likely was this influenza strand not noticed.
Covid-19 has a few strands now, but all are close enough to each other. The Mink one was feared to be too different and be a strand that the vaccine would not capture, but it was contained.