Anti-parasite vaccine but still scratching - Hamster

I took my hamster to the Vet two days ago, because the hamster were always scratching and biting himself so much that he has some areas without hair, the Vet gave him a anti-parasite vaccine and give me a disinfectant solution to put on that areas, which I've been doing. the problem is it's been two days and the hamster is still scratching and biting himself in different areas, what is the problem? The vaccine didn't work? Or it didn't had time to work? Or it's not parasites?

[Dan Sims, DVM]

The hamster needs a dose of Ivermectin to rule out mange and other mites as well.
This is not a vaccine but could be the injection you are referring to.
Call your Veterinarian and ask if he received this drug and if not request it