cVDPV 3 or type 1 poliovirus

I believed even countries maintaining high routine immunization coverage for OPV and IPV also detected cVDPV 3 or Type I poliovirus in enviromental sample, so whether cVDPV or type 1 polio virus is imported or mutated originated strain within the country.

1. If it is imported what could be the mode of transmission?

2. If it is novel mutated strain originated within the country, how?

Can someone help respond?

Thank you