One idea is have it so the vaccine kills off any infection in a persons body at the time its administered, it doesn't provide any sort of antibody to the person so if they are bitten again they will need another dose. The vaccine could also have some negative side effects for a time on the player. Be it a health damage, blurred vision, shaking or slowed movement. This along with a percentile chance of the bite actually causing infection should create times when the player has to really think about if they should take the cure and be vulnerable for a time, or take the chance on their character to keep their faculties.