Who invented smallpox vaccine?

This is an English country doctor, Chen Na. Once, he found a strange phenomenon in a cattle farm: none of the milkmaids died of smallpox, and there was no pockmark. Clever Qin Na thought of China's vaccination method: people who have been vaccinated will not get smallpox again. It can be inferred that the milkmaid may have bovine smallpox and become immune to smallpox.

In order to find out the reason, Qinna observed in the cowshed many times since then, and he found that the milking girl did have bovine smallpox. Cattle smallpox, however, only has symptoms such as finger water sore, low fever and local lymph gland enlargement, which will soon be cured. At this point, Qin Na can preliminarily conclude that people will not be infected with smallpox after they get bovine smallpox. Since 1788, Qin Na has been observing and experimenting continuously for 8 years, and has conducted in-depth research on the symptoms of bovine smallpox. From this, he concluded that vaccination can prevent smallpox.