The flu shot is a vaccine.
Yes, against current strain. When flu mutates, the shot changes too.
The shot you got few years back still works against that particular strain, but not against new ones. You don't get booster of the same vaccine, you get new shot for new strain.
The current shot for covid may have been effective (despite side effects) against original Wuhan strain, but not against delta strain.
Just as delta, this new "omicron" variant got mutated spike proteins that mRNA suppose to tech your body to identify and fight against. Thus, the current so called "vaccine" is useless.