Variolation

It's unclear how effective this was as inoculation, but the basic idea was there, and people did usually develop smallpox symptoms after this, such as a fever and a rash, but fewer people died than if they had acquired the disease naturally.

Variolation wasn't foolproof, but it worked well enough for quite a few centuries.

It traveled outside of China, and eventually it was being used across the whole world.

But then came a man called Edward Jenner.