A 50-year old American immunization technician in South Vietnam was discovered to have pneumonic plague eight days after the onset of fever, chills, pleuretic chest pain, and a cough productive of large amounts of bloody sputum. His survival, untreated, for that period of time with a disease renowned for its rapid lethality suggests that prior immunizations with killed and living vaccines significantly altered the natural course of the disease.
Cohen RJ, Stockard JL. Pneumonic Plague in an Untreated Plague-Vaccinated Individual. JAMA. 1967;202(4):365-366. doi:10.1001/jama.1967.03130170165036