“this is no small pox vaccine, it doesn't work as well as we first thought”
The smallpox vaccines had an efficacy rate of about 95%, so that’s actually similar to COVID vaccines:
https://www.cdc.gov/smallpox/vaccine-basics/index.html
The reason there’s no smallpox is that everyone was vaccinated so even though there was a small percentage of breakthrough, the disease was effectively eradicated.