what should be noted is
- there are several flu virus versions around. The vaccines may or may not cover all virus versions. Therefore you may still get the flu Even after vaccination.
- flu virus versions evolves and changes from year to year. That is why your protection weakens from year to year after you have had the flu.
- vaccines changes from year to year to keep up with changes in the flu viruses.
And that is why a yearly shot of flu vaccine is a way of reducing the risk getting the flu. But there is no guarantee.