Who invented the first successful vaccine and when?

Edward Jenner, an English physician, is credited with inventing the first successful vaccine in 1796. Jenner discovered that cowpox, a mild disease contracted by dairy workers, provided immunity to smallpox. He tested this by inoculating a young boy with cowpox and later exposing him to smallpox, finding that the boy did not develop the disease. This groundbreaking discovery led to the practice of vaccination, which has since saved millions of lives and played a key role in eradicating smallpox worldwide.