"The oral polio vaccine that's used primarily in low- and middle-income countries - it's been the workhorse of this global effort to eradicate polio. But it is a live vaccine. It's cheap. It's easy to administer.

"However, this live vaccine is continued to be used worldwide. And while you're doing that, some of that vaccine has gotten out into the world. And it's mutated. It starts circulating again, just like regular polio. But early on, it's just - it's still a vaccine. It's not dangerous. And then slowly, it sort of regains strength. And they're finding they can actually genetically see this - that scientists can actually trace it back directly to the vaccine. And now these vaccine-linked cases are actually causing more cases of paralysis each year than actual traditional - what scientists call wild polio."

https://www.npr.org/2019/11/16/780068006/how-the-oral-polio-...