I had a small shelter toy poodle and they gave him a shot at the back of his neck on the morning I took him home . He became extremely sick , panting and shaking and drooling. I stayed up all night with him giving small syringes of water and baby food. When I got him into the vet he said if not for that he would have died. He developed A half dollar sized lump on his neck and all of the hair in the area fell out. My vet was always aware of his reaction to "vaccines " and used extreme caution always. I have been leery of shots since then. I can't imagine giving the same dosage regardless of size in humans.