My memory of a small pox vaccination is that some people got ill for a day or two and a unfortuneate few developed extremely serious side effects. I think a few even died.
There was a post on this forum not to far back discussing this point. If I remember correctly, Japan was the last major country using the vaccine. They discovered toward the end that more people were contracting smallpox from the vaccination than those who were contracting it from natural sources and they put and end to their program.