Why are jenner's vaccine superior to previous methods for conferring resistance to smallpox?

The risk of disease transmission is drastically low  with his method, compare to the previous method with great tendency of transmitting diseases,which were lethal to the recipient.

Explanation:

He had noticed that milkmaids were only affected with cowpox,but are not affected with small pox.Thus he reasoned that they must have immunity against small pox.

Through his crude methods he extracted pus from a milkmaid who carried the virus  for cowpox,and inserted this into the arm of a young through the opening created. After latent period of days he exposed the boy to the smallpox,and surprisingly he was not infected,thus the boy developed immunity to this virus which was related to the virus injected.

Thus he was able to develop immunity against the related diseases.

Besides he observed that this boy did not show symptoms of any other diseases or infection.