

“Then” means that you will be doing something subsequently. “Than” means that you will be doing something instead of something else. Normally this is a grammatical nitpick, but it actually changes the meaning of your sentence by using the wrong word.
It was using secretions from cowpox lesions as protection from smallpox. Cowpox is a similar, but much less dangerous pathogen, so it conferred some cross-immunity without risking actual smallpox infection.