That's... how vaccines work. Including the flu jab you so assiduously avoid. They don't pump you full of drug reservoirs ready to be released in an infection, they give your natural immune system a sneak peak of the virus so when the real deal shows up you can get right to fighting it without giving it a head start on wrecking the joint while your immune system mulls over its response.
Edit: I do apologise in advance to anyone who feels a bit personally targeted by any of my responses in here. Anti-vaxxers are the one stripe of conspiracy nuts who really get my goat. Flat-earthers are pathetic and funny, but medical misinformation is actively dangerous to society.