'pro-vaxxer' originates from the prefix 'pro-' (from Latin 'pro' meaning 'for') combined with 'vaxxer', an English informal agent-form derived from the clipped form 'vax' of 'vaccine' plus the agent suffix '-er'.

'vaccine' comes from Latin 'vaccinus' (from 'vacca' meaning 'cow'); in modern English 'vaccine' was clipped to 'vax' (late 20th century), 'vaxxer' emerged as an informal agent noun in late 20th/early 21st century, and 'pro-' was later attached to form 'pro-vaxxer' in public and online discourse to denote someone 'for' vaccination.

Initially related elements referred to 'vaccine' (medical substance) and 'vax' as a clipped form; over time 'vaxxer' came to mean a person associated with vaccines, and 'pro-vaxxer' specifically came to mean someone who advocates for vaccination in contrast to 'anti-vaxxer'.