After inoculation, vaccination using the cowpox virus became the primary defense against smallpox. After infection by the cowpox virus, the body (usually) gains the ability to recognize the similar smallpox virus from its antigens and is able to fight the smallpox disease much more efficiently.

How did they prevent smallpox?

Smallpox can be prevented by the smallpox vaccine, also called vaccinia virus vaccine. The vaccine is made from a virus called vaccinia, which is a poxvirus similar to smallpox, but less harmful.

Are cowpox and smallpox the same?

Cowpox is a viral skin infection caused by the cowpox or catpox virus. This is a member of the Orthopoxvirus family, which includes the variola virus that causes smallpox. Cowpox is similar to but much milder than the highly contagious and sometimes deadly smallpox disease.