Medical Definition of vaccinia
1a: cowpoxb: a reaction to smallpox vaccine prepared from live vaccinia virus that may involve a rash, fever, headache, and body pain
2: a poxvirus of the genus Orthopoxvirus (species Vaccinia virus) that differs from but is closely related to the viruses causing smallpox and cowpox and that includes a strain of uncertain natural origin used in making vaccines against smallpox—called also vaccinia virus
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