They’ve known this for at least 30 years. The US had a rise in cases of polio in the late 80’s early 90’s. This cases were all traced back to the vaccines. Infants were receiving OPV and their grand parents were changing their diapers, spreading it to a generation that hadn’t received the vax or hadn’t been fully protected as parents were. It was such a risk then, that the US started moving towards the IPV because it was a killed virus as opposed to the live virus in OPV. My mother in law was going through chemo when my daughter was born so they gave my daughter the IVP because my mother in laws immune system was compromised.

That they buried these facts is tye bigger crime, because it kept the flow of OPV going and putting people in poor countries who can ill afford it, at risk.