Whether a soldier in s far off land, a child receiving an oral Polio vaccine dose or an Ontario Hydro working taking tincture of poison ivy to reduce symptoms of contact with the plant the effect is the same - an acquired or enhanced immunity to a chemical poison or living pathogen. Sometimes we confer immunity with an injection, sometimes its taken via the oral route. On both cases it's not uncommon that more than one is required.
When did man begin doing this? "1796" as Wikipedia suggests in 2016 ?