My cat, Bobby was allergic to fleas when he arrived here as a kitten 5 + years ago.

I'm not sure what the injection is that you got for your cat.

I took Bobby to an allergist, and gave him desensitizing shots here at home for a year, to get him over it.

You will need to use regular flea prevention on this cat to keep fleas away from him. I use Revolution on my cats with great success. Did the vet discuss anything like this with you?