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.

If the actual full-strength virus hasn't managed it why would a live vaccine be any more likely to? It's not going to be replicating very much- it was designed to be bad at actually infecting people but good at provoking an immune response.

Both are possible at any point.

>Contrast that to a vaccine that turns out to be not safe: administering the vaccine would then just be stopped.

The smallpox vaccine which was responsible for the eradication of smallpox was a live virus and could spread from person to person. And through the course of time, it did mutate/become contaminated. It was originally cowpox, but at an unknown time sometime in the 1800's actually became vaccinia virus.

https://www.cdc.gov/smallpox/history/history.html https://virus.stanford.edu/pox/2000/vaccinia_virus.html

So, the great achievements of the past, such as smallpox eradication were not as risk-free as we perceive them with our 20-20 hindsight.

That's not what those links say:

> It is not known whether vaccinia virus is the product of genetic recombination, or if it is a species derived from cowpox virus or variola virus by prolonged serial passage, or if it is the living representative of a now extinct virus.