Where the heck are you getting that 490,000 number from? The oral polio vaccine (aka OPV/Sabin) uses a weakened form of the live virus, which known to revert occasionally to virulent form.

Here’s what the WHO site says:

“Since 2000, more than 10 billion doses of OPV have been administered to nearly 3 billion children worldwide. As a result, more than 13 million cases of polio have been prevented, and the disease has been reduced by more than 99%. During that time, 24 cVDPV outbreaks occurred in 21 countries, resulting in fewer than 760 VDPV cases.”

https://www.who.int/news-room/q-a-detail/what-is-vaccine-der...