Vaccine a. : Of or pertaining to cows; pertaining to, derived from, or caused by, vaccinia; as, vaccine virus; the vaccine disease.

Vaccine n. : The virus of vaccinia used in vaccination.

Vaccine n. : any preparation used to render an organism immune to some disease, by inducing or increasing the natural immunity mechanisms. Prior to 1995, such preparations usually contained killed organisms of the type for which immunity was desired, and sometimes used live organisms having attenuated virulence. since that date, preparations containing only specific antigenic portions of the pathogenic organism are also used, some of which are prepared by genetic engineering techniques.

Example:
What is so different about this flu Vaccine compared to other Vaccines?

Taxonomy
vaccines, disease, vaccination, microorganisms, PMID, agent, vaccinated, protection, immune response, polio vaccine, smallpox, vaccine development, rubella, risk, immunity, adjuvants, the immune system, attenuated.

Defination from the web:
vaccine typically contains a small amount of an agent that resembles a microorganism. ... The term vaccine derives from Edward Jenner's 1796 use of the term cow pox (Latin variolæ vaccinæ, adapted from the Latin vaccīn-us, from vacca ...