Before Jenner's vaccine, inoculation was being used to prevent smallpox, but this was a risk as each person was receiving the real disease into their body, and if the dose was too strong, the person could develop smallpox and die
There was a lot of opposition to the smallpox vaccine from some doctors who were making a lot of money from inoculation and didn't want to lose that income, and from the Church who did not like the idea of using a disease from cows in human medicine