Note to people downvoting this (because maybe they think it’s anti-vaccine crankery). The attenuated polio virus vaccine can cause polio. The reward far outweighs the risk.

Should also be noted that in 2016 the trivalent OPV was replaced with a bivalent vaccine in an attempt to further mitigate the risk [1]:

> Until 2015, over 90% of cVDPV cases were due to the type 2 component in OPV. With the transmission of wild poliovirus type 2 already successfully interrupted since 1999, in April 2016 a switch was implemented from trivalent OPV to bivalent OPV in routine immunization programmes. The removal of the type 2 component of OPV is associated with significant public health benefits, including a reduction of the risk of cases of cVDPV2.

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

Also note that 'attuenated virus vaccine' means the oral vaccine that is prevalent in the developing world, and not the vaccine that, e.g. EU citizens get, I had to look this up, as it is not obvious.

I'm not sure why you've been downvoted for this–maybe a little skittishness after years of anti-vax nonsense—but my understanding is also that you're completely correct and the vast majority of polio infections are vaccine-derived. The oral polio vaccine is (I think?) the only widely-used vaccine where this is a risk, and it's well-known.

> I'm not sure why you've been downvoted for this–maybe a little skittishness after years of anti-vax nonsense

A tangent, but isn't this a problem? We should welcome skepticism, if we can easily refute it: no problem.

Refute or Affirm with further research, not a PR campaign.

> I believe

Why?

"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

It's worth noting that by eradicating Polio we can also eradicate vaccine-derived polio, as we'd presumably stop vaccinating people once the disease is eradicated, as we did with smallpox.