There is no universal flu vaccine.
The vaccine’s effectiveness is dependent upon what ‘strain’ of the flu is actually going around.
When the vaccines are prepared for an upcoming season, the mfrs have to literally ‘guess’ what it will be. Sometimes they get it right, sometimes they don’t. But it’s best to get the shots anyway if you are elderly.
This year’s flu and last years flu seem to have been more apt to kill younger folks than older ones, for some odd reason.
Maybe it’s because older folks get the flu shots and are more immune?..........................
Having a functional immune system that identifies and neutralizes invaders without getting tricked into killing the host is far from universal.
That would be the selection part of Natural Selection.