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...