There is some chance that a modified live vaccine can mutate into a superpathogen that becomes immediately widespread and deadly. There is a theory that this happened with the feline panleukopenia vaccine and that this became canine parvovirus, with rapid, worldwide distribution of the new pathogen. Something to think about when we put 250,000 human lives on one side of the balance and a big question mark on the other, and telling people that there's no good reason to be cautious.