Fun fact: if you could magically vaccinate 100% of people with flu vaccine each year and the vaccine was only 50% effective, there would be herd immunity and no such thing as flu season.
If that made you go "wat?" then do a little reading on epidemiology (
herd immunity is interesting stuff).
If you don't want to read about it, then understand that vaccines that aren't 100% effective prevent a lot more cases than their percent effectiveness would imply because each prevented case also prevents the cases that would have caught it from them.
Or just remember that even if it doesn't stop you from getting the disease, it reduces the severity and length of your suffering.