The itch is allergic (to mite feces) and can last a disconcertingly
long time after the critters are all dead. New bumps can even pop up
after they're all dead.
Derbac is basically malathion. The next most popular mitocide is
lindane. Both have enough systemic toxicity that you can't use them
every day on every new lesion until you're comfortable, which is often
what you really want to do with scabies.
I suggest you get your doc to prescribe you some Eurax (crotamiton
creme). It's nontoxic and you can just use it all you like until you're
sure it's all eradicated.