One misunderstanding of the flu shot is that it's intended to 100% prevent the flu and data to the contrary proves it doesn't work. Everyone knows about flu virus variability, so 100% prevention is not the goal as it's not possible. The main goal is to reduce overall prevalence, which it does, AND reduce morbidity, which it does and reduce mortality, which it does. Goal 1 is to try to prevent you from getting the flu, which it does for many. If you do get the flu anyway, the goal is that you won't be as sick and won't be sick for as long, which it does for the vast majority of immunized folks who get the flu. If you do get very sick, the goal is to keep you from dying.
See the 1918 flu pandemic. Mortality estimates worldwide were around 20 million on the low end and upwards of 100 million on the high end (obviously epidemiological evidence was tougher to come by in those day). A whopping estimated 50% of the population was infected by some estimates. All flu pandemics since that one have a combined mortality of much less than 10%. Even as world population skyrocketed, prevalence in future outbreaks decreased consistently.