Vaccinia is so mild that it is typically asymptomatic in healthy individuals, but may causes a mild rash and fever, with an extremely low rate of fatality.
The body's immune response to vaccinia also immunizes against smallpox, and for this reason, vaccinia is used as a live-virus innoculator against smallpox. Vaccinia smallpox vaccine does not contain and cannot cause smallpox, but approximately one in one million individials will develop a fatal response to the vaccination.