my immediate question was whether fentanyl is unique in that a vaccine can be developed for it. From [1] we read "Vaccines for cocaine, methamphetamine, nicotine and others have all been tested in rats and, in some cases, people."

Reading further, [1] says these vaccines have not yet been successful.

[1] https://theconversation.com/a-new-fentanyl-vaccine-looks-pro...