Vaccine

google

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late 18th century: from Latin vaccinus, from vacca ‘cow’ (because of the early use of the cowpox virus against smallpox).

wiktionary

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From Latin vaccīnus, from vacca(“cow”) (because of early use of the cowpox virus against smallpox). Compare New Latin variola vaccīna(“cowpox”).

etymonline

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vaccine (n.)

"matter used in vaccination," 1846, from French vaccin, noun use of adjective, from Latin vaccina, fem. of vaccinus "pertaining to a cow" (see vaccination). Related: Vaccinal; vaccinic.