Nature published a brief piece regarding the development of a immunization for the Hunan Plague virus. They note:
Do people develop immunity? If humans do develop immunity, how long does it last? What kind of immune response should vaccine developers look for? How do we know if a vaccine is likely to work? Will it be safe?
These are critical. Immunity usually means that once you have it you cannot get it again. Well not always. Many communities last just a while. Then again the virus mutates. Off we go again.