As for the efficacy of flu shots, the best way to study is to look at rates of infection and morbidity of hospital personnel, who are generally either required or strongly encouraged to get the shot.Or, most likely, they have built up their immune systems from being exposed to so many germs and viruses.
Even a healthy immune system, used to being exposed to germs, can find itself occasionally weakened by stress, etc. Not to mention that some really wicked germs hide out in hospitals.
But, flu vaccination is voluntary for almost everyone.