Some years ago, I was working in an hospital with old persons, and had to get a vaccine against influenza. I never had this virus before, but I said ok, if it could protect the old person I was working with.
And guess what? I got the strongest flu I ever had just two days after, and couldn't go to work for almost 2 weeks. It was horrible, hallucinations and highest temperature I ever had.
I know that when they vaccine you, they inject in you an inactivated strain of the virus, to make you develop the antibodies against it.
But shit, why putting it directly into your veins? Our body has a lot of external natural defenses before the virus can reach our blood. Just let the nature and our body do the work, naturally...

Would you inject into your blood a small dose of heroin, just to make sure you will not do an overdose the day some heroin accidentally finishes up in your blood? Haha...