Who discovered the world's first vaccine? 

English physician Edward Jenner discovered the world's first successful vaccine. In 1796, he found out that people infected with cowpox were immune to smallpox. Jenner then inoculated an 8-year-old boy with cowpox and later exposed him to smallpox, which he did not contract. This marked the beginning of the smallpox vaccine, which was used to eradicate the disease globally.

In summary, Edward Jenner discovered the world's first vaccine, which was used to protect people from smallpox.