Immunization was the process of introducing a vaccine, via inoculation, into an individual's immune system, by the process known a vaccination, in order to develop an immunity against harmful immunogens.
Doctor Roger Korby, who was responsible for the groundbreaking translation of medical records from the Orion ruins, revolutionized the Federation's immunization techniques sometime prior to the 2260s. (TOS: "What Are Little Girls Made Of?")
In 2268, it was discovered on the planet of Omega IV that the planet developed natural immunizing agents to combat the man made virus created during a biological war centuries earlier. (TOS: "The Omega Glory")