'pro-vaccine' is formed from the prefix 'pro-' and the noun 'vaccine'. The prefix 'pro-' comes from Latin 'pro' meaning 'for' or 'in favor of', and 'vaccine' comes via French 'vaccin' from Latin 'vacca' meaning 'cow' (in reference to cowpox-based vaccines).

'vaccine' entered English in the late 18th century from French 'vaccin' (coined in relation to Edward Jenner's work with cowpox). The combining form 'pro-' (from Latin) has long been used in English to indicate support for something; combining them created 'pro-vaccine' to denote support for vaccination.

Initially, 'vaccine' referred specifically to material derived from cows (cowpox) used to prevent smallpox; over time it broadened to mean any preparation used to stimulate immunity. 'Pro-vaccine' therefore evolved to mean 'in favor of vaccination (in general)' rather than only cowpox-based methods.