You don't become a soldier by playing Call of Duty®... and your favorite football team won't start you at quarterback just because you've mastered their offense in Madden Football®. The simulated world of video games is not a replacement for REAL WORLD experiences. So how can you expect an artificial shot to make you REALLY immune to the flu?
Your immune system is intelligent. Every time your body is exposed to germs or viruses in the environment, your immune system detects the invader and begins an appropriate defense. Maybe your immune system creates a fever to make it difficult for the invader to spread, or a general sickness to shift energy away from muscle activity to white blood cell production. Whatever the response, your immune system innately learns and builds on its expertise - growing its ever expanding protective skills from REAL WORLD EXPERIENCE. That's the right way to build natural immunity.
Germs and viruses don't naturally come armed in formaldehyde, mercury, glycol, aluminum adjuvants, gelatins, foreign egg proteins and antibiotics. Those 'video game' bugs don't exist - yet that's the extra insult your immune system must struggle with when you allow a flu shot into your veins... and there's nothing REAL WORLD about that.