The principle of vaccination was discovered by Edward Jenner in 1796, after he injected pus from a woman with cowpox into a boy, and found he became immune to smallpox, a related, more deadly disease.
Vaccination programmes have been extremely successful at preventing diseases, and in some cases, eliminating them altogether. Smallpox, a virus that is estimated to have killed 300 million people in the 20th century, was eradicated worldwide in 1980 using vaccines. Polio cases have fallen from 350,000 in 1988 to 33 reported cases in 2018, and the virus now circulates in only three countries.
Who do vaccines work ? https://www.who.int/news-room/feature-stories/detail/how-do-vaccines-work
How are vaccines developed? https://www.who.int/news-room/feature-stories/detail/how-are-vaccines-developed