"What is vaccine-derived polio?

Oral polio vaccine (OPV) contains an attenuated (weakened) vaccine-virus, activating an immune response in the body. When a child is immunized with OPV, the weakened vaccine-virus replicates in the intestine for a limited period, thereby developing immunity by building up antibodies. During this time, the vaccine-virus is also excreted. In areas of inadequate sanitation, this excreted vaccine-virus can spread in the immediate community (and this can offer protection to other children through ‘passive’ immunization), before eventually dying out.

On rare occasions, if a population is seriously under-immunized, an excreted vaccine-virus can continue to circulate for an extended period of time. The longer it is allowed to survive, the more genetic changes it undergoes. In very rare instances, the vaccine-virus can genetically change into a form that can paralyse – this is what is known as a circulating vaccine-derived poliovirus (cVDPV)."

- WHO

https://www.who.int/news-room/questions-and-answers/item/pol...

That's what Wikipedia says, and I have no reason to doubt it?

We're fully aware the vaccine does cause some polio infections, and the countries that are seeing cases are still in the middle of their vaccination campaigns.

It's a fair statement and question.

Why is it so hard to get clear data on this topic.

Please show me clear data from reputable sources on all of this. It would end a lot of the the vaccine controversy.

Let's take something like tetanus boosters. They are recommended once every 10 years not because you "need" one per se but the cost and expense of running a trial longer than that made it no longer worth the effort.

> Why?

WHO