The medical math behind vaccinations is actually pretty simple. I had a discussion about this some time ago with a friend of mine who was at med school. The details are hazy but basically all the things you consider are
-Number of people affected without vaccination
-Number of people affected due to vaccination
-Severity of affection without vaccination
-Severity of affection with vaccination
Then apply conditional propabilities and factor in the costs & human factors for each case. Simple, really. The health care officials will easily kill 5 people with vaccinations if it prevents 5 000 000 people to get sick with a 0.1% chance of dying. What makes it more complicated is the herd immunity factor and human nature. It's easy to game the system by not getting vaccinated and relying on the herd immunity. I'm not a doctor so I can't judge the pros and cons of chicken pox vaccination but I do believe that the system is not foolproof, people actually make mistakes and companies try to make profits with vaccines. Also, parent hysteria is a factor that must be considered.